The workbench screen is just like any other public screen. The size and resolution is determined by the hardware, up to the limits imposed by the structures used in graphics/intuition. Unless I'm mistaken, the following structures all use signed 16-bit words for width and height (there are some places where unsigned are used): ViewPort, BitMap, Window, Screen...
Without any hardware limitations, those would restrict the maximum width and height to 32768 pixels. Which is big enough, really.
