http://www.foamfloats.com/

That link is to Foam Floats, now for some reason the site is inactive right now but says they plan to be back up in March. Once they are back up you can use their float calculator. Enter the planes dimensions & weight & it will tell you what size floats you need. March isn't far away, so hopefully you'll be able to use the link in just a few more weeks, Good luck