I've been on the hunt for a decent frame to build from. I keep going back looking to this REW Reynolds mixte except it's a 50cm frame and the rider is 5'5"ish.

Would this be on the larger end for fit? I'm just not sure about Mixte frame sizes.

Also, any thoughts on what that little nub is on the seat tube near the BB?

I messaged the seller with other Q's just waiting to hear back.