Epic is a 6mm game. There were a few scale creep sculpts back in the E40k era (97-98) that were closer to 8mm, but all are still technically 6mm.
The only thing GWs done in 10mm is Warmaster and its spin offs.
All the editions' miniatures are compatible with EA. The basing is very flexible and the counts-as rule allows you to use pretty much anything that's out there.