As an absolute bare minimum, you'll want something 100cm x 200cm. And that's an absolute bare minimum. If you can find one/build one and have room for it, get something 300cm x 100 cm. For a home darkroom, I don't know that it is practical/reasonable to have something wider (150cm vs 100cm), only because it gets harder to clean if they get wider (most home darkroom sinks abut a wall on one side, and anything much wider than 100cm gets hard to reach the back side of).