I should be on 57, but enjoy a 50. The stem is one of those old 90* adjustable L shapes that go up and forward about 110mm, and it has a modern mtb long seattube.
The big key is knee clearance, and having a seat tube that goes high enough (old bikes didn't come stock iwth them). Lesser concerns are that a small frame will put you in a more racing position than the bike designers intended, and you may have knee pain if you sit too far forward on saddle and pedal hard.