I dont have firm numbers for you since it would depend on environmental conditions. As an educated guess though, I would say a fucking shit ton. You wouldn't want to be anywhere near the damn thing.

Not much in space; There's almost no matter to convect!

A sports car radiator has about that size and dumps 1 MW without boiling the coolant.

A car's "radiator" doesn't actually lose heat by radiation though. It conducts heat to the air rushing through it. That's absolutely nothing like a radiator in a vacuum.

That's the point. Forced air cooling is way more efficient than radiative cooling.

The question was about comparing the 1400KW of radiative cooling to how much convective coolig you could get from the same radiator on Earth.