just a quick question trying to figure out what size heads the bolt for the exhaust as well as the midpipe are

 

you need to be more specific

 

what size heads? the factory hardware is 14mm head at the manifold, and either 17mm or 14mm at the end of the cat, as they had two different nuts.

if the exhaust has been removed, there is a high probability that the factory fasteners have been replaced, and any US source will be either 14mm or 15mm and the rear could be anything, as its just a nut and bolt.

so buy the whole socket/wrench set