I got 12's on a 254 (+12/-12) and I don't have any problems. I can make my toes drag if I really try to carve hard to angle the board high enough, but it's a freestyle board so I'm never doing any carving like that anyway. Your 11's will be fine as long as your not doing some insane carves. If it's an intro board, sounds like this won't be the case anyway. Should be good to go, but wouldn't hurt to try it out at the shop like you said.
East Coast Park Rat
2008 Burton Twin 157 w/ Rome 390 Boss
2005 Burton Canyon 168 w/ Ride LS
"its ppl like you, that I would "impact damage" your face if i ever saw you
keep your mouth shut. one more time, please and thank you." -user: chingy_