Minute of angle means roughly 1" at 100 yards. (There's room to argue about small differences here, but for most purposes 1" per 100 yards will do just fine.)
I assume that you're asking about the apparent size of the dot. A 7MOA dot will cover about 7" at 100 yards; a 3 MOA dot will cover about 3".
This is an opinion thing, and here's mine, which is based on very extensive use. It looks like the Comp M2/ML2 only come in a 4MOA size now. This should be fine for anything you are likely to want to do with it. Many people who buy larger "dots" find that they have trouble shooting past 50 yards or so because the dot covers so much of the target.
The apparent size of the dot varies a little, by the way, because the dot "blooms" a little at full power and shrinks when you turn down the power.