Newbe question, Mercury Mountaineer max trailer size

My 2 cents: be very leery of anything an RV dealer tells you. Their interest in selling trailers does not necessarily coincide with what's the best trailer for your family. There's a big difference between being able to tow something and being able to tow it safely.
Assuming it's not dry weight, staying under 4500 lbs is smart. As others have said, the short wheelbase of the Explorer is a limiting factor, though not the only one. Personally I wouldn't go much past 20 ft for trailer length.