
Cavall |
Well it makes the distinction of not taking on other shapes or animals like wild shape later goes on to do.
That means since it's telling you what you can't do, everything else is fair game... At minus one level. That's size changing.
Assuming your GM allows gorillas to be in the monkey grouping (I don't even want to get in on that...) a gorilla would be a good large creature and a massive silverback advanced type would make an exceptional huge one. On the flip side many small monkeys could go tiny.