We all have a sweet spot in terms of bike size, that perfect fit where the seat tube length and the top tube length are just right. My question is..... do a lot of you stick to that size and only consider "perfect" fit bikes for purchase, or do you have bikes that vary a little in size. And if your bikes vary a little in size (particularly in the seat tube length) do you feel it is easier to deal with a bike that is a size smaller than your perfect size or a size larger than your perfect size? And what are the consequences with regard to comfort, aches and pains after the ride, safety, etc?

I tend to get a 56cm bike for road bikes. But I have varied from that a bit when the deal is right. I play around with stem length to compensate somewhat. My smallest road bike is a 54 cm (too small), while my largest is 58cm. That reminds me I need to swap stems on that 54cm bike.

One challenge with a lot of vintage bikes is that there is quite a spread between sizes. I see a lot of 21 inch and 23 inch frames, but don't see many 22 inch frames. This means you go from a 21 inch frame (53 to 54cm) to a 23 inch frame (58 1/2 cm). Pretty big jump.

Then you have the variation between manufacturers. My 1987 Miyata 215ST (23 inch) fits like a 56 cm bike, while the 23 inch Cannondale T600 I had fit more like a 60 cm bike (I sold it).

I like vintage bikes, so I end up getting a compromise, rather than the "perfect" or ideal size.

Modern bikes tend to come in even cm sizes so you can really dial in the size. So my modern Giant is a 56.

In shopping for a touring bike, I've noticed a big difference is sizes. Typically, a 54cm fits me fine, but I've found bikes that are a 49cm that fit as well. Of the bikes that I already own, with their varying sizes, the stand over height only varies a could of centimeters.
On question I've had is that even though the stand over height is fine, what about the rest of the bike geometry?

I have pretty much the identical experiences with wrk101, right down to the model(s).
There are, in fact very few frames built for 27" tires available, actually that were even made in a 22" frame. Back then, Co.s jumped a full 2" in choices. Having contended with this years ago made me sensitive to sizing.
Not these days, not with the availabilty in 55 & 56, bikes with 700 wheels. I like a few bikes that have 55 CC, LeMonds with 55CT on account of the long top tubes.
Stems help but using a setback seat post on a 21" bike works for me. In the past and occassionally these days, I take a ride on a 57/58, it's ok. I prefer the others as I like the feel of having a not so big bike beneath me, depends on how I feel.

The "sweet spot" is most definately NOT your imagination. Bike brands vary in that regard as do the way that they're exactly meas. by the Co.s (beyond just CC or CT).

For two seasons I rode a Felt FC whose top tube was about one cm too long and head tube was about 1 cm too short. I made up the difference with a shorter stem and more spacer, and I could have kept riding that bike. Last spring, however, I came across a Giant TCR whose measurements are spot on and for some reason I'm a little more comfortable on it.

My body is a difficult fit most anyway. With vintage steel I've always gone 1-3 cm short on the nominal side, mostly to get a lighter, more responsive frame, and finally in consideration of top tube length. Frames with relatively short top tubes were more plentiful back in the day. With contemporary bikes and their long CF seat posts, I size mainly from the top tube and consider the head tube secondary. And I'm riding bikes that are nominally 2 cm smaller. This image is very close to my personal setuphttps://www.art.com/asp/View_HighZoom...&imgheight=819

I try to stick to 58cm. I have a 56 that I like and it seems comfortable, but if I take it for longer rides >30 miles, my back will feel a bit sore later.

Standover height is not key, but for a tourer, getting the handlbars high enough is. A high bar requires either lots of stacked spacers, an angled stem, a long (quill) stem, a big frame (high standover and long seat tube), or a long head tube with a sloped top tube.

So standover can be an indication of fit. More imortant are getting your butt the right distance back from the BB and as I said, handlebars at the right height and reach.

Funny, I ride a 57/58 to "cool down" after a "long" ride ... the "feel thing", the back.

I purchase based on top tube length and angles. I stick quite close to my optimum for for these two parameters to maintain my comfort and preserve the handling. However, I let the seat tube length fall where it may. As a result, I've got frames with seat tubes ranging from 48-58cm.

I tend to change "sizes" depending on the particular bike manufacturer and the particular use of the bike. My road bikes range up to 8 cm difference in CC (but about 2 cm difference in TT). My sweet spot is about 52cm TT. But this is a personal preference/habit. I do not care as much about CC or CT, as long as there is at least an inch or two of seat post showing for my fit and I can stand over the bike. I use different size stems depending on the bike to get my perfect position. A size smaller is probably a better choice than a size larger (and it is mainly a safety issue, particularly at stops or emergency situations). And if the stand over height is higher that your inseam, that bike is better avoided.

Also, different manufacturers size bikes differently (for example Schwinn was notorious for sizing bikes based on CT) so it is very hard to tell whether the bike will fit you or not.

As others have said, don't worry too much about frame size. Theoretically, I should feel most comfortable with a 56 c-t, but I've got bikes ranging from 55 to 60cm c-t with different stemsizes and seatpost-length, obviously, and they all feel comfortable even if my riding position is quite different from one to the other. Much depends on the use; some feel better for hilly rides, others on cobbles, etc. All this in function of relatively short and 'leisurely' rides. Of course, if you're talking 6 hour rides at competition-speed, things might get a little more critical!

Joseph