just don't get em too big. go for about a '1 finger fit' which means with the liner out and your toes touching the front of the shell there is about 1 finger worth of space beween your heel and the back of the shell.  if you go bigger, it may be a little more comfy, but it'll be real sloppy and could result in stuff like bone spurs etc. along with crappy performance.
Life sucks, get a fucking helmet
-Denis Leary