Like Rebelman, I have a Bandit 6" and have never, ever regretted the purchase. If I got a 12" I would still keep the 6". I gave it a castered wheel jack on it up front so it can be wheeled around by hand. Anything bigger than a 6" and they get too heavy to be able to be moved by the operator. Sips gas. Ya just can't run logs through.
I lay claim to being the smallest and ugliest operation of any of us, but it's a veneer for being extremely efficient, organized and profitable. Recently moved up from being a one-man operation to now workling as a two-man team, so I may not be the smallest any more. But I am still pretty ugly. Very glad my clients don't hold that against me.
Being able to get my rig into small, tight places, close to the drop zone is a major benefit. Being able to unhook the chipper and swing the infeed chute toward the yard gives a direct straight line to the feed chute, as well as being able, then, to walk between the truck and chipper without having to step over the tongue. The Bandit 6" is easy to work on and maintain, easy to change the oil (exactly 4 Qts) and easy to access and sharpen the knives.
Bigger chippers certainly have their advantages, especially for bigger crews, but then you've got bigger trucks, bigger overhead, bigger fuel costs, etc.
Rent a few different sizes. That's what I did. It all depends on the type of tree work you do. I'm 80% prunes, so all my material is pretty small. On takedowns, I have guys chompin at the bit to come haul away firewood. I have very little need to chip big material, therefore little need for a big chipper. I just need one that is extremely dependable, easy to maintain, easy and safe to use (even for first-time operators) and willing to eat brush all day long. I'm entering my 7th year with my 6". Just adore it.