Yup, biggest you can afford, and that your truck will pull. If I had to do it again, I'd buy a deck over that was at least 14', preferably 16'. Then built my own ramps ( longer ) to load equipment easier. I thought I would be 90% hauling equipment/10% hauling material. It ended up the opposite and it sucks having the bed height the same or lower than your wheelbarrows, and not being able to dump a good pile.
Or just have two....
01 Ford F350 4x4 7.3PSD
04.5 Dodge 2500 4x4 5.9CTD
12 Stealth 8.5x20 Enclosed Trailer
10 Load Trail 7x14 Dump Trailer
Toro Grandstand 52 23hp Kawi
Toro G3 60 34hp Kawi
Stihl MS192T 021 MS260pro MS260 MS460magnum
Miller Trailblazer 275DC
Miller Dialarc 250