I see you are in MI - probably some hills. A minivan for #3500 is probably on the edge of being comfortable on hills and at highway speeds. I always like to have more then needed just in case of bad weather, high winds, etc. We have a 6x12 and we usually have it pretty full. It's a single axle #3500 trailer and we tow with a Suburban which tows it easily, but have ran into snow and wind where I'm glad we had it. Especially on those 8-9hr trips. I think this is a case of no matter what size you pick, you will find stuff to fill it
Recommend one with brakes even at the lighter weight. Ramp doors and shelving definitely help. Make sure not to put to much weight at the rear - they can sway out of control if there isn't enough tongue weight. Also things like tie down hooks on the floor, places to hang items of the walls, interior lights all help. We've been considering installing solar panels - enough to charge robot and tool batteries while on longer trips.