He's quoted as using 10s sizes 10-52 strings on this forum somewhere. A lot of people prefer 11s for dropped C but do what feels best to you!
I can imagine that 11's would be OK for mostly rhythm playing but a little stiff for playing stuff like Vai. Just found this post by Mikey on an old thread:
On most of Steve's guitars, he uses Ernie Ball Super Slinky 9s - 42s, (with the hot pink packaging.)
Every once in a while, Steve puts 10s on a guitar or two, especially when he wants a lower tuning. 10s are on one or two of his Universe guitars right now because they are tuned down a whole step, and the low B string is tuned another whole step further.
So I presume he also used 10's to record Bad Horsie seeing as that's in a lower tuning.