Each model should be mounted on a base to determine the width and depth of the unit. Each Troop Type has different approved base sizes, which are listed below.
In some cases, you might be using a model that requires a larger base than described for its Troop Type. This is completely fine, as long as you try to keep it as close as possible to the normal approved base sizes. However, you may never have a base size that is smaller than the approved base sizes listed here.