What do you mean by "intensity"? If you mean average flux (flux divided by area) then the coil should be just large enough for the magnet to pass. The magnetic field is strongest inside the magnet, as you can see by drawing the field lines - they all have to pass through the interior of the magnet, so they are very dense there.

I don't see what intensity means either, but you'll also arrive at Greg P's answer if you try to maximize the induced voltage. This is proportional to the rate of change of the flux through the coil.

The field lines inside the dipole magnet go one way, and the return flux outside goes in the other (remember, magnetic field lines are closed), so they cancel if both go through the coil.

Since the Magnetic Flux is defined as an integral over the surface area the larger the area the better, However this is assuming that the magnetic field is constant (or constant enough) over that surface area. If you are using a bar magnet I would say this loop needs to be like Greg P said roughly the same diameter as the magnet. If you get any larger than that the field will not be consistent within the loop. And if you get any smaller you can't position the loop in the strongest part of the field.