[whswhs]

GURPS Powers recommends that a power should have two attacks, two or three major defenses, two or three major movement abilities, two or three mental abilities, and an unspecified number of physical transformations. If you call the last "two or three" again, you're looking at around a dozen abilities. So I'd say that maybe ten to fifteen is the boundary between "a short list" and "a long list": Go over that level and your modular ability gives you more options that an entire power.