Since Bhell posted an excelent answer that deserves feedback, I am posting this "answer" of mine with some observations. I think it could help people with the same doubts, in the future.
First of all, I enjoyed the pragmatic approach to bike fitting, instead of a somewhat questionable numerology I've been seeing around lately. Also, the rationale starts from the problem up, and not from a canned solution down. Now let's go piece by piece, considering this one frameset:
Primarily, a bike fits if you can position pedals, saddle and
handlebars relative to each other so that they match your body's
proportions. (...) That includes e.g. toe clip overlap, stand over
height, bottom bracket height, heel/panniers clearance, steering
geometry, or that an extremely short or long stem gives a weird
You got it right from the start: when I went over the bike to ride, and after some minor adjustments (none of them at the extreme of adjustment range), I felt the bike almost exactely like the previous one, except for a slightly long reach, which I still don't know is due to too much saddle setback or too long stem. Nothing to be too much concerned, since I rode at most 2km and the immediate feeling was very soothing and familiar compared to the previous 56cm bike.
About other characteristics you mentioned, there is no overlap (plenty of clearance, actually, which is good for randonneuring with fenders), BB height was fine (quite nice fixed-gear cornering), pannier clearance is yet better (44cm chainstays instead of the vanilla 42cm of my other utility bikes, good for fenders in the rear, too), and the steering balance is stable without being too slow or hard to quick-dodge.
The critical point is often if you can put your handlebars of choice
in the right spot, too. (...) normal hand position is on or behind the
brake lever hoods with a drop bar, and not on the straight section
next to the stem eye [like a flat bar]. If one does not want to resort
to extremely short or long stems, a frame usually only allows for a
good fit with either drop or straight bars.
I've been thinking about this, too. The stem on the photo is 135mm horizontally (center to center), and although it has helped me along the years with smaller bikes, specially when mounted pointing up, I think this bike should have a smaller stem if I put a flat bar, and a yet smaller one if I put drops. Since the current stem is long and I feel the bike only slightly longer than needed, I think there is room for adjustment without resorting to extreme stem dimensions.
(...) the classic Raleigh road geometries do not have overly long
reach, and the stem shown on your picture is way longer than what I
would consider aesthetically pleasing on a frame like this (especially
with quite little seat post to be seen).
I haven't looked at it from this angle, but this sounds compelling. I have to admit that I plan to inflict some "oldschool-performance" upon my fellow riders, since the tend to look down at "old big steel stuff" from their aeroplastics bikes. For a better effect, the classic non-slope lugged kit can be complete with corresponding short stem and short post...
I'm pretty sure that the fork is not original. Ask the seller if the
original still exists. It probably is a threaded fork, which allows
for higher building quill stems. And it probably looks nicer, too.
(...) The fact that the frame was built with a classic quill stem in
mind, but has a threadless fork today, does not make it easier: It may
be that you can't rise the bars enough with an ahead stem (and the
fork already being cut).
Gotcha! The fork is the original threaded one, and the ahead stem is mounted to a threadless-to-quill adapter. The end result is very firm and has quite acceptable looks. By the way, the fork is PERFECT because it is curved, sturdy, has fender eyelets and gives the frameset a very nice handling (not to mention the fact that frame and fork were designed for one another by a reputable manufacturer).
I think that the average 56cm frame is quite a bit too small for the
average 1.88m male (horizontal top tube, or "virtual seat tube length"
I can live with my other 56cm frame becaused I mounted it fixed-gear for training, so a more sportive position is fine if I go with good speed and not too long. For touring, randonneuring or even commuting with more gear in the winter, I have had serious problems with sore hands and feeling too cramped over the bike "in the long run", so that's why I looked for a larger frame, although I didn't have something THAT large in mind.
In the end, I decided to keep the frame, and here goes the summary of the story, which includes the insights provided in Bhell's answer:
Why I wanted a new frame:
- To have a more relaxed geometry than I currently have with bikes that are relatively small for me;
- To have a bike faster than a city bike but not so stripped down as a fixie, allowing to achieve relatively high cruise speeds for long times and some comfort.
What interests me in this frame:
- It has good design (a classic road frame from a reputable manufacturer), good construction (4130 lugged chromoly tubing, fender brazeons) and good value (relatively low price due to usage signs which don't interfere with function).
- It's high head tube brings the upper headset cup much closer to what I have been trying to get lately (I've been replacing my other bikes' stems and handlebars because with age coming I starter to feel they were too low).
What was potentially problematic with this frame (and prompted me to write the question):
- "Ride feel" could be cumbersome because some unexpected effect of larger geometry and size;
- Its reach might be too long (although I already wanted longer reach);
- Its inseam might be too high;
- The correct seat height might expose too little seatpost, thus making it hard to put some taller saddles like sprung Brooks and the like (not really a plan, but one never knows).
Why I KEPT the frame after all:
- The Bottom-Bracked contact point (the only fixed-position one) has a good height from the ground, not so low as to hit the pedals neither so high as to make it hard to put the foot on the ground;
- The Seat contact point position could be naturally found with a good range of adjustment in every direction (both horizontally and vertically);
- The Handlebar position got almost right (perhaps a bit too long) with a flat handlebar and a relatively long stem, which makes me think I could either shorten the stem a bit and keep the flat bar or shorten the stem even more and put a drop;
- If I decide to put a drop, I'll get a good upright position in the flats, a nice cruising position in the hoods, and a usable (not too low) position in the drops. Although the bike begs for this, I am particularly fond of flat bars because I have always ridden mountain bikes, and the brake levers being always at hand is a thing I value very much, specially while riding in a fast tight pack.
- The interference problems (top tube against crotch, inner thighs rubbing the upper seatstays a bit) didn't bother me after I quickly got used to them. Actually, it is cozy to have a bike so much "closer" to me (I'm not trying to be vulgar here, you know...).
As a bottom line, it was fundamental to have the points of view of everyone who answered or commented. I suspected that here in StackExchange people would be more traditional and classically oriented, and the vast majority supported the idea that big is not a problem if you don't make it into a problem. That also, reinforced a perception I have, that all this hype about smaller-frame-size-possible due to weight or responsiveness or center of gravity or whatever is way too much exagerated.
Thanks for the help, and if I have some useful additional feedback I'll post it.