Some of Neil's comments in Becky's Thread made me start thinking about this, and if it helps some of my fellow Clydes/Clydeettes save some money, so much the better.
Background, I'm 30 with a lifelong history of nagging back/knee problems, mostly caused by accidents and my weight. and 6'1" with short legs, long torso
When I started riding again last summer, (After 14 years or so away from bikes) I started at 280lbs, down from my worst at 305-310 on a 23" (58cm) Schwinn Continental, the conti, didn't fit me very well at all, It was all I could do, despite being a fairly "Active" clyde to get my leg over it. So I went looking for 21" frames, there being a shortage of older 22" frames to be found. I ended up with two pretty nice road bikes with 21" frames, both were pretty cheap, standover gave me about an inch before hitting anything, top tube length was pretty cramped, but I rode mostly on the flats of the bars and bolt upright, so it seemed ok. Also, I never raised the seat high enough to get full leg extension, (my knees seemed happier that way,) and I was/am a noob with no clue about proper fitting to begin with. So at best, my riding position was pretty inefficient.
Now, since last fall I've been watching what I eat, quit drinking anything carbonated, cut back on the baked goods and all the trips to DQ, and I've rode a lot, 20-30-40 miles at a time. I was really surprised that my rebuilt left knee could take more than 10 miles! But.. over the past few months my noob mistakes began to come to light. The stronger my legs became, the more they supported my bad knees, so I began to raise the seat every now and then, (not even thinking about it really) A few weeks ago, the cramped position on the small frame really began to affect my back, even with a long stem. And the reality that a 6'1" man does not belong on a 21" frame became very clear. I must have looked like a circus bear on the 21" bike. The standover on a 23" frame is not as tight as it was before, (Gravity?) so I'm moving to 22"-23" 56-58cm bikes now. I've been lucky enough to find reasonable inexpensive replacements.
Just making a short story long,