If it's Quigley pre-05*
, it should be 8 on 6.5" (Factory Ford), ditto SMB. After '05*
the Ford Quigleys will be 8 on 170mm (SuperDuty Ford Pattern). It wouldn't hurt to double check with the spare before buying something expensive off CL.
As far as rim size if you have 16.5 you should be safe with 16" on up, and that and the width can be determined by your tire choice.
*Edited: I thought it was pre-08 based on Quigley's sale of the last 8 on 6.5 stuff, but see Switchfoot and SheepShaggers posts below for better info!