This is simply the wheel arrangement of the locomotive. 2-4-2: 2 wheels on the front, leading pilot of the engine, 4 drive wheels, and 2 wheels on the rear, trailing truck.
The minimum diameter will be listed elsewhere, We use diameter, not radius, in O-Gauge 3-rail. This is usually o27 on traditional sized products, but can be up to o72 on larger or more scale sized models.