I think our mirrors are a little bit smaller than law says they need to be, but I don't think they will get that picky on you if you are being safe.
I have one that I stick onto my existing mirrors on my challenger, that is 4"X8". It jiggles too much to see clearly through it, but it brings us up to "legal" status.
The wife can tell when I go down, and she keeps an eye on me enough to see when I "thumbs-up" her.