how do i set the pie menu radius for one pie menu not for all?
Afaik you can’t.
Blender uses a global radius value for all pies through context.preferences.view.pie_menu_radius.
You’d have to temporarily store this value somewhere, set your radius, launch your custom pie, and finally restore the old value. It would probably involve wrapping the pie menu in a modal operator using PASS_THROUGH.
Happy Pie Day! (Sorry, couldn’t resist.)