The external size of wrenches can vary quite a lot. Ring spanners tend to be the slimmest but you need enough room to get them off the end of the bolt once it is tight. You can also get ring spanners with slots which fit over a bolt shaft but theses are a bit more chunky.
So really you just need to measure the size of the wrench you want to use.
Having said that you might want to consider using a captive nut, this will make assembly easier and you may well find that it is more stable and distributes the load better for this sort of installation in wood. It doesn't take much for a hex nut to dig into plywood.
Indeed you may find that 5mm plywood is really a bit thin to use this sort of construction method at all.