There's really not a distinct limit it Gandalf's Power, when he was Gandalf the Grey he didn't have that much power, and he couldn't match against Saruman the White.
Basically all the Maiars are equal in power, it just depends on how they use that power or if their time is here to fulfill their purpose.
In TT: EE, Gandalf says that he was Saruman, as what he should've been. Gandalf's saying that he represents how Saruman should be. Not the corrupted Saruman at that point in time.
Sauron was also a Maiar once but he grew power hungry and he got to how he is now.
Hope that answers your question. 🙂
Yeah, and I think Gandalf the White still has some limitations, to think that the Valar are still superior over them. Though that doesn't make him very weak either. We just doesn't see any eyecandy kind of powers. Tolkien's magic wasn't about fireballs and lightning. Quite often, the "visible" magic Gandalf does is exactly just lighting up his staff. Tolkien perceived magic as a presence, something you can feel but not see. Gandalf uses a lot of magic in all three movies. Jackson respected Tolkien's vision: magic is a presence, not a firework.