Any question answered...

Fit's not so crucial,as

1. You're probably not going to be doing massively long rides on it
   
2. by the time you've ficked about with aerobars, saddles and rails, stems etc, you can lose 2 cm either way here and there without much issue.
   
   

That said, if you are XL size, then XS isn't going to do it for you, but the difference between say 56 and 58 isn't going to kill you. Not a lot of handling involved.