The Polypro has really replaced neoprene vests as the best way to add some winter warmth to your wetsuit (and will add loads more warmth than a standard rash vest). Something like a lightweight stretch jersey the polypro has a similar lining to the 'firewall' found in expensive suits and will keep you in the water longer worn under your wetsuit in harsh north european winter conditions.

Sizing

Your rash vest needs to be a tight initial fit to be most effective.  It will loosen as time goes by so a good fit is important.

Small:           37" Chest
Medium:       39" Chest
Large:          41" Chest
Xtra Large:  43" Chest