Not sure what size it is, but it should be a standard hex head bolt.
Not to mean or condescending in any way, but if you got a decent socket wrench set for your garage, it wouldn't matter what size the bolt is, because your set should probably be able to cover it. I understand a reason to ask is to be sure your socket set has the size that you'd need, but if your set goes up to 19mm, it's probably as big as you'd need for basic car work. Only things like drive axle nuts would require something bigger.
I've always found it pretty amazing how capable a decent SAE and metric socket wrench set is for doing things for the car or house. I've done pretty extensive things, such as brake jobs, changing out suspension (springs/shocks/swaybars), changing out exhaust systems, bolt on engine mods with a basic socket wrench set.