Size of a Magnetic field?

Hello! I was just wondering, how can one determine the size of an electromagnetic or regular magnetic field?
For instance, if I had a regular bar magnet and I wanted to calculate to what distance the magnetic field could attract/repel another magnet, how would I go about doing that?

I know the measure of force would be Tesla, but the size of the field itself is what I'm not too sure about...

Thanks!