Of course it depends on where it's centered but I can tell you a 7. anything earthquake is considerably large and can be felt miles and miles away from its center.
What I always find amazing is, you can have a large aftershock that isn't considered another earthquake but rather an aftershock from a previous earthquake. Not only that, but aftershocks can come years later from the original quake. Some years back we had a fairly large quake (6. something I think) and later in the news they said it was an aftershock from the Whittier which was in 1987.
Go figure.
I assume since they labeled this latest Japan an earthquake that it is separate from the original. Either way, poor Japan. I feel very badly for that country.
TIS