a 2x3 will be fine on 2x6 floor frame. I would use the 2x6 for the floors if you have it it will be that much stronger!!!
if your thinking of going any bigger you might wanna put supports under where the evap will go. I have a 2x6 evap and used 2x10 16"centers for floor frame and doubled up on the 2x10's under the evap. just in case.