To sum up some of the last few posts: You should ignore what length the seattube is on MTBs. Just check standover and most importantly Effective Top Tube length.

Regardless, 19" is probably too big for you unless you find a bike with an extremely short TT vs ST.

I have an 18" with a 56cm ETT and a 19" with a 60cm ETT. You'll be pretty darn stretched out on either one of those.

I also have a 15" XC bike with a 56cm ETT, so you'd likely want the 13" iteration of that frame.

This assuming your road bike range of 50-53cm is on square road frames.