It depends on how complex a shop you're talking about.
For 99% of day-to-day on-field repairs, I just need a tool kit the size of a loaf of bread. There's really not that much on a paintball gun that needs, say, a long pipe wrench or a floor jack or an oxyacetylene torch.
My typical kit for a full day at the field- at least back in the day- was basically a specialized set of hand tools. The usual allen wrenches and standard open-end wrenches, various pliers and screwdrivers, all the way down to specialty stuff like swiss needle files, an eXacto, an 1/8" pipe tap and die, and a set of quality electrical pliers mainly used for the shear which can cut machine screws down to size.
On top of that, I'd have a little bin-box full of various O-rings, screws, a few basic hose fittings and plugs, and whatever repair parts might come to mind, like Tippmann detents, 'Cocker LP hose, generic Nelson springs and - back in the day- Automag reg seats.
Using all that, there was very little I couldn't fix at the field.
Of course, there's lots more that can go wrong that can't be fixed with just an O-ring and a bit of teflon tape. Like stripped screw holes, broken feed necks, bad pin valves...
With not much work, one could put together a surprisingly complete mobile workshop in a van or even something like an Explorer or Blazer.
I've thought about it off and on over the years- we once even looked at an old city transit type bus not only to carry all the field gear (we didn't leave it at the field) but also as a sort of team bus and mobile workshop.
Down there in the States, it might be a fun idea. Bud Orr, I'm told, got his start by towing just such a shop-in-a-box trailer to the local very popular field and doing both repairs and sales. Up here in Alaska- even up in Anchorage, let alone down here- it'd be kind of a waste.
But just for the sake of the argument, a mobile shop would be easy. As I've said before, in my early days, I had very little- a Jet mill-drill, a small lathe, a bare handful of cutters and drills, a few hand tools, and a bench grinder-slash-buffer. There's no reason that couldn't be put into a full-size van, or better yet, a small box van.
Add in some storage for spare parts, jam a shop compressor in there, a good-sized tank of CO2, a big vise on a sturdy work table, and a small tool chest of hand tools. No sweat.
I remember back when Smart Parts was starting to circle the drain, they put one of their tourney trailers up for sale. Big semi-trailer type thing, one side opened up to reveal sales counters with glass display cases, and inside was a workbench with a vise. That'd be far more than enough room for both a workshop and a retail store.