What size fuse should be used for 2 amp power supply?

If the 2 amps is the output amperage of the power supply, the maximum that should be drawn from the unit is 2 amps. The load amperage that is connected to the power supply should govern the amperage of the fuse used. There is not much range there, the fusing could go from .25 to 2 amps.

If the input amperage is 2 amps then the input and output voltage of the power supply should be stated.