It's in the help here:
@JeremyWright I have a few follow up questions:
the help specs it as "Crown Width" so no it is not radius
height is above ground level
as you can see from the latest doc we have deprecated these layers in 3.0 and later releases.
switching to a standard feature layer will give you more control over the tree representations and parameters as well as their attribute-driven properties, and you can still use the nice tree models included with ArcGIS Pro styles.
This quick youtube tutorial might also offer some insight: