I just picked up a 1988 Trek 400T. It's in good shape and I got it for a really nice price.

The lady I bought it from is approximately the same height/proportion as me and she's had the bike for 20 years and she says she's perfectly comfortable on it. I think it's too big for me. After dropping the saddle, I'm comfortable riding the bike- to the pedals, to the handlebars- all good. However when I stop, I'm effectively sitting on the top tube.

I'm 5'8" and wear 30" trousers.

What size bike should I be riding?