I like how you're thinking...Blackstone. Have you checked out Tventuring forum? and CompactCampingConcepts ? I would say minimum size is a 4'x6' trailer. Smaller may be a bit unstable and your RTT will hang over too much. Where do you want the kitchen? Off the back or a pull out. My trailer is 4'x8' (no RTT) and I have a slide out near the front (tongue) where I have the cooking supplies. I also added a folding counter top on the side to create an "L" shaped work space. If this is only a chuckwagon, 4'x6' is big enough to carry all food, water, propane, and cooking supplies for a large group. If you do all your cooking on the Blackstone, you'll still need a burner to boil water.
Good luck and have fun!