Snapperhead

She's 5'6" and has about a 78 cm inseam (floor to crotch). She's riding an older 16.5" Trek mountain bike but I'm looking to take some Ultegra components from another bike I have and put them on a frame for her. What size frame should I be looking for?

Leisesturm

Building out a bare frame correctly is not a rookie assignment. How come the rookie question? Sorry, I couldn't resist breaking your caged bearings because this is the Internet. You gave your wife's height in Imperial Units why did you give her inseam in Metric Units? I'm going to assume that you don't intend to put Ultegra components on a mountain frame. If the only baseline example you have is a mountain frame I would just forget it even exists and start from scratch. The Competitive Cyclist fit calculator is the closest thing to a Gold Standard for getting the kind of info you seek that I know of. They want a LOT more than just your SO's inseam. You probably know that. So.... what are you waiting for, get to measuring. Some guy's find measuring their SO's for new frames kinda fun. I for one am not going to cheat you out of marital quality time by doing frame size guesswork on your behalf.

H

debit

OP, I'm the same height and have the same inseam as your wife, and I ride anything from a 51 to a 54. The calculator at Competitive Cyclist put me at 52 for a French fit so you might be safe assuming that. But I strongly second the suggestion to get her measurements and run them through yourself.

ETA: I mostly ride a 54 and a 53 frame. The 54 is a WSD and feels perfect. The 53 is a Jamis Aurora and even after putting on a shorter stem and adjusting my saddle as far to the front as I can (and still have proper pedaling position) I still feel a bit too stretched out.