I currently use 11's but they seem a little loose.
Whatever you feel is best. If 11's feel loose, use 12's.
I just use the lightest gauge I can without bending the strings at all while fretting them. so I usually tune down to D standard with 9's. My strat is almost always in Eb standard and I use 9's for that as well.
I'd use 13's. Might be a little loose though.