Is my St. Bernard Small or big?

My 7week old male st. bernard is 10 inches tall and 5.25kg or 11.55lbs.Is he small or big for his age?

It is hard to tell, because puppies are like children. You might have some that are big as youngsters, then don't grow much. Or late bloomers, who suddenly shoot up like crazy as they hit general adolescence. Just feed you dog GOOD QUALITY puppy food for large breeds (no cheap stuff like Alpo!), and ask your vet if you should taper into regular puppy food or adult food. There is suspicion that if a puppy has HUMONGOUS feet, he will grow up pretty big, but it's hard to gauge on purebreeds that are generally all supposed to be big. Better for unknown mutts.

I would say a bit on the small side. I had a litter of Australian Shepherd/Chow/Newfoundland/Rottweiler mixes and they were between 7 and 9.5 lbs at 6 weeks old. They weighed from 64 to 110 lbs at maturity.