The head is 9/16, which is 14.3mm.... of course wrenches won't be exactly either one, since that would make for a press fit.
I dug out my old set of (3/8 drive) sockets and have tried sockets from both it and my metric range using whatever feels the most snug fit. It's when I encounter a missing bolt (battery retainer) or need a longer bolt for some purpose (eg pulling harmonic balancer) or other pulling task (eg steering wheel) that it falls apart.
On anything else I'm working on (that is metric throughout) I have a stock of metric bolts (and taps) at hand to at least identify the thread even if the bolt isn't suitable (wrong length, needed elsewhere, etc). From there I can order what I need from a bolt stockist. It's that that I need to bring myself up to speed on with the Suburban. I'll get there!