Does size matter?

Yeah the reach is pretty important but this is measured as bottom bracket to handlebar on x and y axis.

Many say to size a compact frame based on the virtual top tube length. This is wrong as unless you know the seat tube angle and how it affects top tube length the measurement is meaningless.

some good advice there napalm. bar height is certainly hard to fix if it's wrong, especially on a threadless headset. The reach of a bike is important too, you can compensate using a longer/shorter stem but you can quite seriously affect the handling if you go to extremes.

This is how I see bike fit...

You determine how high your saddle should be and how much setback you need.

You determine how much reach you need to the bars and how much drop.

Saddle and bar position can be measured in relation to the same fixed point. The bottom bracket.

Correct saddle height and setback can be obtained on a large vairety of frame sizes through height adjustment, saddle rails and various setback seatposts. Watch for the seat post angle to determine wether the desired setback can be achieved with any given seat post. For example personally I cannot achieve correct setback on a 72 dgree angle even with a zero setback seatpost.

Correct bar height postion however can not be so easily compensated for. A frame too small will result in a bar too low, a frame to big will result in a bar too high. Spacers and flipping the stem can only compensate to a point.

Therefore it is my opinion that the most important measurements on a compact frame is the head tube length and seat tube angle.

Determine the saddle and bar position in relation to the bottom bracket on an x and y axis. Then determine whether a given frame size can allow you to achieve this postition.

I love compact frames 55cm seat tube and a 57 top tube for me keeps me in aero but my winter bike is traditional tour geometry and a 57cm top tube but long seat tube - makes my nads sore and making some adjustments.

Best thing is to get a bike fit - and go from there.
I'm getting myself fit then another bike fit before buying a frame.

Mike Burrows takes the same line as Ron Cooper on this - he always intended the compact bikes he designed for Giant to be only offered in S, M, or L for exactly that reason. He maintains that the vast amount of the population can get a bike to fit by swapping things like stems, and bars, using seatposts with different amounts of layback etc.

i've got to say that over the years i've ridden bikes that are the 'wrong' size with a bit of tweaking more often than I've ridden one that's the right size - with no ill effect. Although I'm pretty sure that riding with handlebars that are slightly too wide for the last few months is reponsible for the pain I get between my shoulder blades when I've been out on the bike for a while.