next time your trying find out nut size take a crescent wrench and get it as tight as you can on nut. Now slide it off and with a tape measure measure the opening. That should give you what you need.
The exhaust will have to come off or to the point you can swing it out to pull old cover. Put masking tape over chrome cover so you don't scratch it putting it on.
Please add your state and model of bike you have. Click user CP @ top of page and then far left click edit signature. Thanks