If regular condoms are tight move up a size. Trojan makes standard, a Large size, Magnums and Magnum XLs. The Larges are probably a waste of time, they're just very slightly bigger than standard condoms, but if you're really concerned about not buying something too big, try that as a stepping stone. If they're tight, go for the Magnums; if the Magnums are tight pick up the XLs.

And since nobody else has said it yet: http://www.theyfit.com/