The contractor I was working for in WA used a 5x8 behind a 96 tacoma 4x4. He had some racks in it, and it would haul most of the stuff we needed on a day to day basis... chop saw, table saw, compressor, various air and electrical tools, plus some suppplies (solder, glue, teflon, plumber's putty, etc. etc. etc.). Good size for this use... easy to get on and off the truck, can be moved by hand when off truck.
I'd recommend he get the barn doors, rather than a ramp. Easier to use in tight areas.
I've got a 7x14 wells cargo that I got to carry my bikes... special ordered w/ single axle and v-nose, but I can hardly move it when it's not on the truck. My Ford Lightning ('99) pulls it with ease (5k towing capacity), but I'm also careful about staying out of the boost when pulling it.