Sizing question for Colnago Prestige

Hi,

Looking to get into cyclocross. A friend of a friend has a 2012 Colnago Prestige for sale. It's a size 54cm. I'm 5'11", with a 34" inseam. Any thoughts? He lives in a different city otherwise I'd try it out.

Colnago Prestige Geo:

http://www.colnago.com/prestige/

I see Nys is 5'11", rides a 56cm, with his seat setback pretty far, and a 120mm stem - so I'm assuming I'd need a 56cm. But, maybe his positioning on the bike is way different from mine? Thank you for your input.