Well, you could have been a bit more specific in what exactly you were doing when you got that error message, but I'm willing to bet that you were entering a mesh size less than 1. If you want to enter an element size of say half (units are irrelevant here) and you type in ".5" then you'll get that message since it doesn't recognize the dot as a part of a number. Simply type in "0.5" and the message disappears...
Irritating, yes... I think that has been fixed in the more recent service packs...
jt