Old Epic is 6mm. Aeronautica Imperialis and Adeptus Titanicus are 8mm scale which is what Legions Imperialis will be.
As I've mentioned before, I have pretty much everything already for AI and AT, so I'm very much looking forward to LI as are my friends in our gaming group. They also have a lot of AI and AT so being able to use our models in another game is a bonus for us.
Hopefully when it comes out next month (probably releasing at or around GenCon is my thought) my friends took the hint and will be chipping in to get me a copy.