I think I would definitely want one bigger than the biggest tank I would be dumping. Probably not a good idea to overfill one and make a mess.
I saw one guy have a empty tote in the back of his truck bed and used one of the 12VDC FLOJET 18555-000 Waste Water Pump Kits to empty his waste into the tote. When it finished he removed the hose from the trailer side and tied it off high on the truck and drove to the dump site. There he let it empty the tote using gravity flow.
I thought that was neat (no mess) idea...