>In terms of energy density, you can just increasing the speed can't you? //
In which case you only need a tiny mass to start with ...
The speed limitation is presumably going to come in with the rate at which you can alter the magnetic field to still accelerate the mass. Also you'll get drag as it impinges on local fields (Earth's magnetic field) and there'll presumably be eddy currents and local electrical fields to cope with too which will become more significant at higher flux rates.
In which case you only need a tiny mass to start with ...
The speed limitation is presumably going to come in with the rate at which you can alter the magnetic field to still accelerate the mass. Also you'll get drag as it impinges on local fields (Earth's magnetic field) and there'll presumably be eddy currents and local electrical fields to cope with too which will become more significant at higher flux rates.