What size suits?
With walker wheels, the general rule is the larger the wheel, the easier it is to push, especially over rough ground. Walkers with small solid wheels are really only suitable for indoors.
Three wheels or four?
Walkers with three wheels are more maneuverable but less stable than those with four wheels.
Four wheel walkers are easy to push, especially if the wheels are large and swivel. This is a real advantage.
If you need to lean or push against the rollator's frame for support, you may find a standard walker to mobile - the frame may "run away" from you. The solution may be a wheeled walker with slow-down brakes, where with the turn of a knob you can adjust to have tension on the wheels.