At the sale barn where I work we have a customer who pulls a 6'8" x 36' with a Ram 5500. He loads it full and goes. We also have a custom hauler that had a 7' x 40' that he pulled with a Ram 3500 - he likes to brag on how much he can haul with it, but I've never seen it with more cattle in it than you could've put in a 32'... We see every combination you can imagine from half tons pulling 24's to one tons that never pull more than a 16', and about any combination of big trucks and trailers as well.
All that said, later model one ton diesels are rated to safely pull a loaded 6'8" x 32', and I'd consider that the most desirable combination for custom hauling with a light truck/gooseneck.
By far the most popular trailer size these days is 24'. Mostly you can pull them with a 3/4 ton, and they're reasonably easy to get around in most places. Many people around here built their driveways, gates and load out areas with 24' trailers in mind, and they're often hard to impossible to get anything much bigger in and out of - so that might be something to consider if you're planning on custom hauling.
