Hi Derek,
I live in southern Cal. in the Inland desert region. It gets pretty toasty here as well (but it's a dry heat;^)).
I don't have AC, and haven't used one, but have two guys on my field that have and use the portable units. They both say the unit is effective, but isn't a great way to go unless you have the room for the cooler as well as whatever baggage you may have.
As far as size, the ones they have are the size of a medium cooler that one would take to the beach without getting a hernia. Sorry but qt. size of the cooler is not available to me here.
Judging from the opinions of these two friends, I would say go for it and get as large a unit as you think you can reasonably use on a consistent basis, given your normal mission profile.
Good luck, Mike