I purchased a used construction 240V space heater (4.8KW) 12/2 cabtire wiring (rated 20A).

Can I connect this to a 208V supply or must I re-size any items in the space heater

A 4800 watt unit will draw 20 amps at 240 volts, and 23 amps at 208 volts. Either way, this unit must be fed by a 30 amp circuit with all #10 wire.