At the moment I have a 40", 700 kg scrap-magnet dismantled here as a challenging repair job. It has the typical coaxial construction, a flat steel top plate 2" thick, a 12" diameter central pole with the other pole as a 2" thick annulus. The coil is inside, covered below by a non-magnetic plate, welded slightly inset, between the poles. Protection of the coil is very important. The non-magnetic plate used was often second hand manganese steel, light armour plate, about 1/2" thick.
Magnet coils can be wound for any voltage over about 50 volts, usually about 300V. Typically the DC supply will come from bridge-rectified three-phase 400V. That must be reduced, once the typically 50 amp current is flowing, to prevent overheating of the enclosed coil. The biggest challenge comes when turning off the current to kill the magnetic field quickly. The magnetic stored energy must be reliably dumped into a load. Failure of that load can result in a voltage spike that destroys the coil insulation.
A 4 foot diameter magnet will not start to pick up light pieces of steel until it is about 2 feet away. Any steel picked up that rests against the poles becomes a magnetic pole extension and so increases the range. The picture in berkeman's post above shows the way that chains of small scrap form between the poles. Those chains do not form until very close contact is made, typically 1".
The idea that a magnet can pick things up that are more than a "pole separation" away are fanciful.
Maybe you should explain what you are really trying to do.
