Browsing Marketplace I see this old Bridgestone BB-1for a decent price. Says it's 17" frame. Seems they use inches for MTB sizing and it always confuses me. This bike looks fairly small right? Looking to build out something new for the wife who's about 5'3" with shorter legs. It has 26" wheels. Getting confusing info on 17" sized bikes. Any thoughts?

The BB--1 was Bridgestone'seconomy version of their CB city bike series. It was introduced in 1992. I'm assuming the 17" dimension is the seller's measurement as Bridgestone literature shows frame sizes in centimetres. Here's a link to the 1992 catalogue, which has geomtery charts for the BB-1. They're even kind enough to include standover heights.
https://sheldonbrown.com/bridgestone/1992/1992.pdf

17" is on the big side for someone 5-3 with short legs. I have a couple of bikes for my 5-3 wife (who does not have short legs). One is 15 inch, the other is 14 1/2 inches. Both are MTBs. a Paramount Series 70 and a 1994 Trek 950.

yeah, not sure where the seller is getting their info on 17". I know according to charts 17 seems big but this bike sure seems to be on the smaller side according to the pics.

According to the 1992 Bridgestone literature, the smallest, non-step-through BB-1 frame was 42cm, which equates to 16.5".