Know this thread is a month old, but I'll suggest the game with the best "summoner" class to date. It's "Champions Online" I was able to make a custom class that could summon 9 "permanent" pets that follow you everywhere and act semi-autonomously. They are all about your size, but if you are willing to lose one that follows you and instead make it stay within a summoning circle, they can be 12x as big as you.
I "LOVED" that custom class, one man raid party. Sadly the gameworld itself is unbelievably boring to me. And I hate the comic-book art style used, gives me a headache. But damn did that game let you make ANYTHING you wanted character-wise.