I am 5'11" and my target size frame is 57cm. For the last year though, I have been riding a 52cm Nishiki frame for commuting. I can get enough seat post extension and with GB All-Rounder bars (with an 80 or 90 mm stem), it actually is quite comfortable. I use to think (early 80s) that I needed a biggish frame and rode a 60 cm Palo Alto frame for quite some time. I was talked into trying some smaller frames and found that after messing with seat positions and stems, they were actually much more suitable for me. I don't think I am disproportional, but everybody's different.

The International is a very nice ride so it is worth trying. After a few hundred miles you can decide.

Good luck.