(sorry, for all the confusing numbers but this will be the most expensive component on her bike and I want to make sure the size is right!)

Okay, finally ready to buy my little sister's Soma frame. She decided on a Soma ES. She test rode a Surly Cross Check 46cm at the shop (effective top tube length of 528.8mm and and a center-to-center top lube length of 515mm) That was fine for her, but Soma has a few more sizing options.

So for the Soma ES, the 46cm has an eTT of 510mm, the 48cm has an eTT of 520mm, and the 50cm has an eTT of 530mm. The 50cm Soma ES, I guess, is closest to the 46cm Surly Cross Check but that size Cross Check had a sloping top tube and my sister is a little on the short side, about 5'2, with a 29.5 in-seam. So my thinking is this, a lower top tube on the smallest ES, the 46cm, would be best for her. The stand over height is 29.2 inches. The 48cm ES has a stand over height of 29.8 and the 50cm ES has a stand over height of 30.0. I think a 30 inch stand over is too tall, even those the 50cm is closest to the eTT on the 46cm Cross Check. So it's between the 46 and 48cm Soma ES. Which one should I pick? Feel free to correct me if my understanding of any of these measurements is wrong.

edit: I think the cockpit sizing can be adjusted with a shorter or longer stem. It really is a difference of 10mm. So I could get her the smallest bike possible and just get a stem 10mm longer? Is there any downside to that?

Have you tried any sizing calculator? I'd recommend Competitive Cyclists calc, female French Fit to start with.
My understanding is that for more relaxed long riding a larger frame is better, for more aggressive style a smaller frame may be better. I also think that taller frames look better, less spacers. Though overall difference seems to be really small, not sure if it will matter in this particular case. You can adjust the reach not only with stem but with the compact handlebars too.
Standover will be close to the maximum with 48cms, but myself i'd give it a try.

hm, bump. I kind of don't see any disadvantage in the 46cm. The measurements including the chain stay length are all the same. You say a larger frame is more relaxed. By larger you mean longer chain stay as in touring bikes, right? The head tube angle goes from 73 to 72, though. I forget what effect that has. Anyway, the 46cm Cross Check uses a 75mm stem. I assume a 46cm ES would call for the same, but how would using a 95 or 100mm stem affect handling, if at all?

edit: thanks for the sizing calculator suggestion, but I think I'm in the right ball park going off the Cross Check test ride, just debating the trade off between a taller stand over height and a shorter eTT / longer stem.

I just finished building up a Soma Double Cross. I ride a Specialized Roubaix in 56 and a medium frame GT 29er. I ordered the Soma in 56. The frame is a bit taller than I wanted. I only have a few inches of seatpost showing and will need to get a bit shorter stem. I guess you could say I now have a "French Fit", but it doesn't quite "look" right. maybe go one size smaller...

I just got a hybrid built on a Soma Double Cross frame, and the fit is almost perfect but I can't get my saddle back far enough. Even with an offset post, I find myself wanting the saddle back another inch. The biggest comfort/efficiency factor on my last bike was the same. It comes down to the angle between my hip joints and the pedals, when the seatpost is at the right height and my back and arms are comfy. Which is a long way of saying, I wouldn't count on being able to fix a fit issue with a longer stem. It's all just so personal... That's my two cents as a complete newcomer to the forum. Good luck!

The thing about small and large frames is more like the rule of a thumb, you always have to study the geometry charts. Soma offers a bit more sizing steps than many others, so in this particular case the difference doesn't seem to be too big, you'll probably be on the safe side with both sizes.

No, I meant that a taller frame would normally allow a more relaxed ride.

Given the fork rake is identical 73 degrees HT will make the steering a bit more "snappy" and will put you a bit more forward. The longer stem will shift your weight forward too, making your ride a bit more stable (or sluggish .

Thanks, Mikhalit. That's just the information I was looking for.

I think it would be exceedingly odd, especially for a woman, to not be able to stand over the frame or be able to get a toe down from the saddle. To my mind, those are far more important considerations than 10mm of reach one way or the other, especially when fitting women, who seem to be less tolerant of being stretched out. Go smaller, go shorter. No doubt.

Can you try to locate a bike with a 30 inch standover and get your sister to try it. That's a good way to figure this out.
To a small degree you can lengthen the top tube with a longer stem. Again it would be nice if she tried a bike with that much TT length.

Perhaps she should go to a bike shop and get a fitting. That would eliminate a lot of guesswork.