Effective TT length of a 52 cm '05 Roubaix is 53.7 cm. http://www.specialized.com/us/en/bc/...1&menuItemId=0
You didn't say what year your friend's Dolce was, but effective TT length of a 51 cm Dolce has been 51.8 cm for model years since 2006. In 2004-2005 it was 52.1 cm.
Effective TT length of a 54 cm '09 Dolce is 53.5 cm. So if that's too big for you, the Roubaix is even a couple of millimeters longer.