For a heated bed that large, you might want to get a big plate and stick 4 of the 120/240VAC silicone heaters to the bottom, with a solid state relay to control them.
No matter what you're going to be using a ton of power, since energy in = energy out, but at least that way you don't have to worry about converting that amount of wall power to 12/24VDC before it gets spent.
If you're going with a glass surface, it might not be a terrible idea to have an aluminium (or copper) plate sandwiched in the middle to help keep the heat spread evenly, too, though you'd have to be careful the different thermal expansion doesn't crack the glass if it's attached too tightly.
Another thing to consider, stepper drivers do have a maximum step frequency. If your microstepping is too high it might take a long time for the print head to move that distance, regardless of what the motors can handle.