Homunculus Servant as a level 2 Artificer, how big is it?

For context I have been reading forums and stuff, and I read a few different websites, including this one, but it might be in a book I don't own. So just asking, how big can a Homunculus be? I know some people say the size of a housecat, or 2.5 feet in a square. But does it say that somewhere on one of the books or is that just something everybody agreed on and is still technically homebrew?

Also, that space is the area the creature controls, not necessarily it's actual size.

Keep in mind to that Goliath should be considered large, as they are right on the boarder, but are handled as medium for game play reasons. Horses are large, and occupy 2x2, when they’re technically a 2x1 by mass. 

So there usually is some fiddling room within a size class to how big it actually is.