I think the question deserves some additional treatment as to frame sizing. I realize the OP may have downplayed aspects of frame sizing in his question, but that's arguably the wrong approach. It benefits the end user to know how to select a bike size, and the parameters I describe may not have been in common use in 2011, when the question was asked. They are now in very common use. Thus, although the approach I describe may be a bit difficult for totally new riders to just pick up and use de novo, I believe the question is worthy of a new answer.
Basics of frame sizing: stack and reach
The image below is from a Road.cc article. It shows the two most important parameters in sizing a bike, stack and reach. Both are measured from the center of the bottom bracket to the center of the top of the head tube.
Reach is the bike's x-dimension, and stack is the y-dimension. Different types of bicycle, e.g. touring bike, hybrid, endurance road bike, road racing bike, will have different ratios of stack to reach. For example, given the same overall size, the average endurance road bike has higher stack and shorter reach than the average performance road bikes (here, I mean bikes oriented to road racing, rather than endurance riding), and the average touring bike will have higher stack and shorter reach than the average endurance road bike.
However, within a certain sub-category (e.g. touring, endurance road, cross country MTB, enduro MTB), manufacturers may differ. Some may make longer and lower frames than other manufacturers within the same category. To further complicate things, the relative fit may change across bike sizes. For example, considering endurance road bikes, which are designed to cover long distances comfortably, and not necessarily to race. Bike Insights, which collects and analyzes bicycle frame geometry, rates the 2020 Trek Domane in its larger sizes as somewhat upright, and very upright in the largest size. In contrast, Ribble's (a major UK online bike shop with a house brand of bikes) 2020 Endurance Ti Disc is rated as very aggressive, meaning longer and lower, across all its sizes.
As discussed in the road.cc article and this Cyclingtips article, the frame stack and reach don't include the headset and spacers. External headsets cups can add quite a bit of stack height, and riders can further modify the actual riding position using spacers. You modify the reach by selecting a stem length, and secondarily a stem angle. I think that -17 degree stems were common among classic road bikes, but they're rare now. More common angles are 10 and 6 degrees; you can flip the stem for added stack height (although now your reach changes). Additionally, you can select handlebars with greater or lesser forward reach, although handlebar selection also needs to consider your hand size. These items control the adjustable portion of your riding position, and many riders may make small adjustments as they go by based on other riders' feedback and personal intuition.
Once you know your current stack and reach, and your preferred handlebar position (i.e. total height of spacers under stem, stem length, handlebar reach), this makes it easy to estimate if a new road bike will fit you, and it enables you to guess what parts might need to change. For example, when I bought a gravel bike last year, I compared the frame's stack and reach to my own, and I was able to specify that I wanted 10mm of spacers under the stem and ask to change from a 100mm to a 120mm stem.
To my recollection, stack and reach figures have become commonplace within at least the last 5 years. When I started riding in the early 2000s, I don't recall them being discussed at all. You would size a bike by its top tube and seat tube lengths, and perhaps the seat angle. My custom road bike was built in 2007, and its original CAD diagram didn't discuss stack or reach at all. At the time the original question was asked in 2011, I can't recall how commonly stack and reach were used, but they may not have been that common.
This may be too much for a totally new rider, but riders with unusual physical proportions might throw the Bike Insights categorization off. My custom road bike is rated as very aggressive for an endurance bike (it's a private entry, so other users won't be able to see it on the site). However, I have proportionately short legs and a long torso. Thus, my actual riding position is, I think, moderately aggressive at most.
Saddle height and setback, seat tube angle
Stack and reach describe the bike's sizing from forward of the bottom bracket. One other set of measures needs to be considered related to your preferred saddle position. Before you set your actual fore-aft riding position, you need to get your saddle's height and fore-aft position set first. You would move the seatpost up or down to adjust height, and you slide the saddle's rails in the seatpost to adjust the latter. If you need a further aft position than your rails allow, you may need to find a seatpost with more setback. To my knowledge, seatposts with setback figures ranging from 0 to 20mm are quite common. Set forward seatposts may be available but they are rare; triathletes may have used them at one point to help modify the riding position on standard road bikes. Seatposts with over 20mm of setback should be rare, and I can't name any off the top of my head.
I suspect most riders will be able to raise or lower their seatposts enough to get their correct saddle position on a bike that's roughly their size. Very occasionally, some bicycles might have features that interfere with this, but I suspect this doesn't affect the majority of riders. For example, Trek's higher-end bikes have seat masts, and they specify a certain minimum and maximum saddle height for each frame size. To my recollection, I would otherwise prefer to ride their second-smallest frame if not for the seat mast restriction. I believe this stems from my proportionately short legs. I have far less seat post showing on all of my bicycles than any peer riders who aren't on level top tube frames (i.e. more classic styled road frames).
The bike's seat tube angle, pic courtesy of Ride Far, may be a consideration. A steeper seat angle will put your saddle further forward if you leave it centered on the rails. The bike's seat angle will limit the extremes of the position you are able to put your saddle in. That said, I suspect most riders may not need to worry about seat tube angles, although some may prefer slacker or steeper angles.
I am an edge case as to seat angle as well. I prefer a very steep seat angle. Many stock bikes in my size have 74 or 74.5 degree seat angles. My custom road bike has a 76 degree seat angle. I tend to rule out bikes with 73 degree seat angles, because there is not enough adjustment available on the saddle rails with many common seatposts.
Seatposts come in different amount of setback, which enables you to further adjust your saddle position. The picture above shows a zero, standard, and extreme setback post, although note that this is for a triathlon bike with a proprietary seatpost. On road bikes, setback from 0 to 25mm is common.