The 'main' portion of the M416 is 4x6, the top a bit wider.
I have a fairly heavy lid on mine (made from ash lumber), and have an RTT on top.
As far as size, it's perfect. I usually have it about 1/2 full for a 10-12 day trip (that's when I can get groceries every day or two)
When going somewhere that I won't be close to stores, I still have a little extra room inside, even when carrying enough food and ice for 2 people for a week long trip.
I tow it with any of my Jeeps (CJ and TJ now, used to tow with the XJ before I sold it)
I have 31x10.5-15 tires on steel wheels, and had no problems towing it to Utah with my CJ. (OK, I wasn't the fastest guy over the divide on I-70, but I think my 32 year old CJ would have been just as slow without the trailer! :coffeedrink: )
I think that a 4x6 is a perfect size to tow behind any SWB rig.