Currently if I select a circle the radius value is highlighted by default in the "Edit Definition" window. Is there any way to change it so when I select a circle the diameter value is highlighted instead?
I do not believe so. If you hold Shift and hit the Tab key, it will become the focus, but there is no way to make the focus the default for the command. That would require logging an incedent report with Siemens if you are on maintenance.