DR.1 has it right. The landing gear has no width requirement. You can use a single wheel under the fuselage with outrigger wires or wheels to keep it from dragging a wing. Normally, though, the landing gear should be wide enough to keep the airplane from being too "tippy" when on the ground.

A tri-gear plane needs the mains just aft of the balance point so that it will keep the nose on the ground at rest, but still be able to rotate for takeoff. Put the mains too far aft, and the model won't rotate, and a gear that's a bit aft of ideal will tend to hug the ground until you put in a lot of "up" elevator and then it will "leap" into the air with a too-high nose-up attitude.

Tail-draggers need the width the same...so the plane's not to "tippy", and have the axles of the main wheels at about the leading edge of the wing when the plane's in the flight attitude. Slightly forward of that won't hurt.

In all cases the model must track dead-straight when allowed to roll. The mains must be absolutely parallel to start. Toe-in or toe-out is a controversy best discussed elsewhere.