Create a RoundLeg Design that mixes in the Cylinder Design. Remove BaseAssembly from RoundLeg's Design. (We're going to proceed as cleanly as possible from here on out: Cylinder contains BaseAssembly, so it's fine to remove BaseAssembly from RoundLeg. There is no harm in keeping it, but removing the duplicate Mixin saves a tiny bit of overhead.)
Add a Cached, Parameter rule to the RoundLeg Design called size and make its formula 1.
Next, override the radius rule from the Cylinder Design Mixin. The new override rule is not a Parameter rule and its category is not Inputs but Geometry. The override has the following formula:
this.size / 2
Now the RoundLeg takes a "size" Parameter that will define the radius of the leg, just as a "size" Parameter drives the length and width in the SquareLeg Design.
