I agree with others that the most important component of fit in selecting a frame is the top tube length, and also the seat tube angle. It is obvious why top tube length is important, as it is one of the main determinants of reach. STA is a much overlooked determinant of reach, but an increase of only 1 degree in STA increases the reach by 1 cm, assuming a constant KOP position. A common problem in small sized bikes built around 700cc wheels is the top tubes are proportionally too long to fit the large wheel, hence the reach is too long. So, you need to figure out what reach you need and work back from there. While you can deal with a too long top tube by using a short stem, if the stem gets too short the handling becomes twitchy. I think seat tube length is one of the least important factors in selecting a frame, provided you have enough clearance to comfortably mount, dismount, and not be hurt in a fall. In the old days having too much seat post showing was a problem cuz you couldn't get your stem high enough (the problem rivendell cites), but there are now stems readily available that angle up as an alternative way to get enough height. You can easily swap out seat posts and stems, but you can't change your seat tube angle or top tube length. Also be careful when comparing frame sizes between manufacturers, as bottom bracket heights vary, as do whether the seat tube is being measured from center to center vs center to top. I can fit into bikes ranging from 44-51 cm depending on model. To determine your size in any given bike, you need to look at the published geometry, choose the frame with the reach parameters based on the published STA and TT length that are desirable for you, and then look at the standover height to see if its in a range you can fit with available seat posts and stems. I also look at the published fork trail, as they indicate the handling. I like a trail value in the 5-5.5 range. Many small framed bikes have trail values exceeding 6, which makes for sluggish handling (in an attempt to avoid toe clip overlap, they sometimes put on a fork with large rake or increase the head tube angle, which pushes the front wheel away, but gives awful balance over the bike). So, without even test riding a bike, by looking at these tables I know if it can be made to fit me and how it will handle. BTW, I am 5'4", 30.5" cycling inseam, and I ride a 44 cm (center to center) Terry Titanium Isis with a 73 degree STA, 48 cm TT, 24" front whee, 700cc rear wheel, angled stem that projects ~10 cm forward, and nitto noodel bars. My 44 cm Terry Classic touring bike is set up the same, but with a 49 cm TT and 9 cm stem. I like having the 700cc rear wheel since casettes are designed to give optimal gearing for a 700cc wheel, but the 24" front wheels let me get the reach I want without any compromises in bike handling or toe clip overlap.