This type of addon relay is intended to be mounted to any convenient existing bolt that you have available. As there is no standard for what size bolts to be used in an engine bay or around the body, there is no standard size recommendation for mounting this relay.
What you will do is find an existing bracket or mount in the engine bay, then back the bolt out and stack this relay against the bracket, putting the bolt back through. You can often find a spot on the sides of the strut tower that will accommodate.
One thing to keep in mind about these relays, is they are can be sensitive to water and excessive heat. Avoid mounting in "splash areas" and near the exhaust manifolds to prevent premature wear to the relay.
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To make the install a little nicer you can use one of these relay sockets instead of spade terminals.
Slip a piece of heat shrink tubing over the end of your wire and over the end of the relay socket's wire, connect the two wires with a crimp on butt splice, then slide the heat shrink tubing over the end of the splice and heat the tubing to seal it all up.