Depends on various factors, length of arms, body legs........ I ride a 56cm modern Basso road bike but in square steel I ride 23-231/2"
There are a couple of threads on here which have links to a size calculator and tips on sizing....
The upshot is that you need to sit / ride something you are going to buy.
What he said.
I would add, as an mtb'er recently turned roadie, as well as the seat tube, you need to pay attention to the top tube length.
Like you, I'm 6'1" and as good luck would have it, my first purchase fits well, but contemplating a second bike (one is NEVER enough), I took some measurements and compared them to a couple of my MTB's. The big difference with a road bike is the reach.
With MTB's you are using straight or nearly straight bars, maybe with bar ends, so you have top tube, plus stem to give you your reach. On a road bike, many people, including myself, ride on the brake lever hoods and they are an extra 100 to 120mm beyond the stem.
Just another factor to bear in mind.