Going to a smaller wheel increases torque and decreases speed on the hub motored wheel.
If your wheels are 700, go 27.5, or if its 27.5 go 26, or if its 26 go 24 or 22 bmx.
Having a 26 hub motor on a 700c with 700 rear, was fine for me, has been for a year now.
Play around with the trip sim and motor sim