Here's a link to a multi-brand sizing chart:
http://www.coloradocyclist.com/help/sizingCharts.cfm
Regarding buying a larger size, I bought some Pearl Izumis on the internet. They are a little large for me. But because you close them with 3 adjustable velcro straps, they fit tight and snug everywhere but the toe box. No problems. Better too big than too small.