Here's a simple Action that you can define in your process (make it a "UI Action" by setting this field to True).
Action Name: Set size to match effort
Applicable to: Task
UI Action: True
Having done this you can expand your task hierarchy in the explorer, select the tasks you wish to set size on, and then right-click and select UI Action -> Set size to match effort .
Note: From version 2.9 of xProcess this action is included in the Simple Process.