Here are the two options using the standard size modifiers. Note that enlarge person does not grant the usual size bonuses, but a much weaker change. Therefore, I'd prefer Size Change to Change Size.
Change Size (Sp): At will, a weisshund can magically increase its size. This works just like an enlarge person spell, except that the ability can work on the weisshund. This is the equivalent of a 2nd-level spell.
Size Change (Su): At will, a weisshund can change its size in a fashion similar to the effect of an enlarge person spell. The weisshund gains a +8 size bonus to Strength, a –2 size penalty to Dexterity (to a minimum of 1), and a +4 size bonus to Constitution. It also suffers a –1 size penalty on attack rolls and AC due to its increased size. The weisshund's natural armor bonus improves by +3, and its claw damage improves to xdx, while its bite damage improves to xdx. The weisshund may remain in Large size for x rounds. Returning to its normal size is a swift action.