What is largest heat bed and source

[old man emu]

I'd say that a 600 x 900 mm print bed is something that you are going to have to make yourself. This is not as hard as it sounds. The biggest problem is sourcing resistance wire fine enough so that you aren't using heaps of Watts when heating. You will need to know the resistance per metre of the wire to calculate the power per metre (P = V^2/R)

If you can source the resistance wire, then all you need do is get a sheet of aluminium (aluminum to Yanks) and lay down a layer or two of Kapon tape. Lay down your resistance wire in a coiled pattern. Cover the coil with more Kapon tape, or a coating of heat-proof fibreglass resin.

Old Man Emu

EDIT: I have a better idea. Instead of Kapon tape, apply a thick coating of ABS slurry to your sheet of metal. Then lay down the resistance wire and cover it with another layer of ABS slurry. Since the print bead is not going to be heated much above 120 C, the slurry will not melt.

For your calculations, the resistance per metre of Nichrome wire is 5.4 ohms/metre for AWG24 (0.020") and 2.16 ohms/metre for AWG 30 (0.32")

OME