My choice would involve keeping the bikes in an enclosed trailer at night. You can't beat the security of having everything hidden and locked up, plus it keeps the bikes and gear out of the elements.
As previously mentioned, the towing capacity of your vehicle, whether it be an RV or minivan, is really the driving factor here when sizing the trailer. Your owners manual should tell you what the capabilities are.
A larger trailer will hold more stuff, but will also weigh more and have more wind resistance, putting more strain on the tow vehicle. If I were using the minivan to pull it, I would probably go with the smallest enclosed trailer that would fit the bikes and gear while staying within the load ratings of the van. I think I'd opt for being able to pull easier at faster speeds and better gas mileage over having extra storage space.
That said, I pull a Pace Shadow 8-1/2'X20' behind a GMC Duramax diesel truck. The trailer frequently has four bikes, a couple of quads, gear, and enough other stuff to pit three bikes during a desert race, (EZUp, spare wheels, dump cans, tools, etc.) When the trailer isn't full I usually wish I was pulling something smaller.
Another thing, enclosed trailers tend to hold their value pretty well, particularly the better brands (Wells Cargo, Pace). If you got a small one now, then later ended up with an RV and the desire for more space you'd probably be able to sell it for close to what you paid for it if it was in good shape and get a larger one.