The "effective top tube" length (ETT) cannot be taken in isolation when comparing bikes. It is the combination of the seat tube angle (STA) and the ETT that determines whether the bike will fit you or not in reach. You want your body to be positioned properly in relation to the bottom bracket (BB) first, which is the primary concern when fitting a bike. From there, the other measurements effect the fit as follows.
If you were to take two small frames with the same ETT, but one with 74 degree STA and the other with a 76 degree STA (a common range for your size frames), the fit would be very different. Let's look at a couple of examples:
- If the bike with the 76 degree STA puts you in the proper position relative to the BB with the saddle positioned in the middle of the rails, the bike with the 74 degree STA would force you to slide the saddle forward to position yourself properly over the BB, effectively shortening the top tube length.
- Conversely, if the bike with the 74 degree STA puts you in the proper position relative to the BB with the saddle positioned in the middle of the rails, the bike with the 76 degree STA would force you to slide the saddle backward, effectively lengthening the top tube length.
This is why bike manufacturers have started quoting a measurement called "Reach". This is the distance between vertical lines passing through the BB and top tube/head tube intersection. If you take two bikes with the same Reach measurement, as long as you can get your saddle positioned properly (by changing the amount of seatpost offset or position or by sliding the saddle on the rails) they will both fit you in terms of reach.
It helps to know how much of a difference a degree in STA makes at the saddle, which varies depending on your saddle height. My girlfriend is 5'1" with relatively short legs and rides the same size frames that you do. A 1-degree change in STA is ~7mm at the saddle for her, so that would be a good guide for you.
You also really have to take manufacturer's stated frame sizes with a grain of salt, as they don't all measure them the same way. Some are measured from the center of the BB to the center of the top tube-seat tube junction (commonly called "center-to-center" or "C-C"😉 and others are measured from the center of the BB to the top of the seat tube (known as "center-to-top" or "C-T"😉. Some with sloping top tubes use an "effective seat tube length". It's a mess and it can even vary within the same brand or bike model. In my girlfriend's case, her new 46cm Cannondale SuperX is actually slightly smaller than the previous year's 44cm bike. Go figure.
The bottom line is that when looking at frame geometry, you should concentrate on either the Reach or the combination of ETT (which you are doing) and STA, then check the "Standover Height", which determines whether you'll be able to straddle the bike comfortably. As I'm sure you know, that latter dimension can be a real deal-breaker for petite people.
I hope this information is helpful. I may have given more of an explanation than necessary, but I have no idea how familiar you or others who may read this are with frame geometry terms and their meanings.
