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Is it practical to get a slightly larger sized bike and thereby make the seat lower relative to the frame in order to make the bike less aggressive? I'm looking at a Specialized Allez road bike and it looks slightly too aggressive for me so can I get one size larger?

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Buy the bike that is not only the right size for you, but the right geometry. If the model is too aggressive, then look at another bike with a geometry more suited to you and your riding aspirations. A bike fit can adjust many things in a bike and set it up to suit you best.

All that said, many people are in-between two bikes for sizing. If this is the case for you, then going to the larger size is the right thing to do. Ideally you can try both sizes (for decent length rides, not a loop around the shop carpark) and choose based on how the bikes feel.

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    Re: "going to the larger size is the right thing to do" unless the larger size provides zero (or "negative") clearance to the top bar?
    – Sam7919
    Aug 11, 2022 at 20:33
  • @Sam Good observation. You can adjust a lot of stuff on a bike but unless it's a steel frame and you've got a very delicate touch with a sledge-hammer, adjusting the top tube is generally out of the question!
    – SSilk
    Aug 11, 2022 at 21:53
  • You and me have a different concept of "between sizes" if the ride ends up with 'negative clearance"
    – mattnz
    Aug 12, 2022 at 1:17
  • @mattnz I see what you mean. Makes sense. There is always this little puzzle. It is perfectly possible to ride a bike that would collide with one's "region" if the bike is vertical, provided every other fitting dimension is good. One "just" has to remember to slant the frame every time one mounts/dismounts. But it may not be quite sensible to do it.
    – Sam7919
    Aug 12, 2022 at 2:25
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This is actually quite commonly done in bike fitting circles but you need to be aware of the stack and reach measurements. The reason it works is that going up by one size can increase the stack height by up to 30mm on some bike models but may increase the reach by only 10mm, or even less in many examples, which is easily made up by adjusting stem length (and most people change from the stock stem when fitted properly anyway).

Basically, you need to compare the geometry tables and ideally try both sizes.

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Don’t. You’ll just end up needing a short stem, you’ll have less standover height, less compliance in your (now shorter) seatpost and a heavier frame.

The Allez mostly looks “aggressive” because all the photos on the internet seem to be with zero (or very few) headset spacers and the stem angled down.

If you want/need a more upright position, try to get it with the steerer tube uncut and use as many spacers as you need. You can also turn the stem around to be angled up or get a stem with even more angle.

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Oh yes that's a very good strategy.

Most road bikes have the handlebars way too low. This, combined with the short range of adjustment (usually you have only 15mm or 20mm of spacers plus the opportunity to flip a 6 degree stem giving 20mm extra so you only have 35-40mm of height adjustment range), means that there's no way using the stock components to have the handlebar at a reasonable level.

However, today frames are "compact" with sloping top tube. This means standover clearance is almost never a problem even with slightly too large frames.

So your options are:

  1. Buy a frame of the correct size and buy a high-rise (17deg or 35deg) stem
  2. Buy a frame that's slightly too large and buy a shorter stem

I use the second strategy.

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