I'd say personal but large enough to cook for a party. Portability isn't necessarily needed, but i think pretty much any design could be tweaked to sit on a towable trailer. I'm also thinking of a deeper unit with a rib-o-lator type setup. Similar to a heating oil tank, but laying on the flat side.
"Eating 'Que isn't making me fat- it's the beer I drink while lounging by the pit."