The airstream KnowledgeBase is just threads broken down by airstream model. We hear you about size, but in some ways size is even more of a premium with so many people-- either way, every inch needs to be accounted for, with special consideration given to sight lines and other design elements to feel as spacious as possible. Our 86 is a narrow body, of course. Fully 14' of the coach is the bunkroom and the side bath, so that leaves the rest for living space and our own bed, plus eating space for ten to twelve and sleeping space for grandma. You have great ideas and are paying attention to the most important part: design and layout. 20' can live much "larger" than twice as much space if it is a well-designed 20'. The sofa we put in our 32 is 9' long, 2/3 of which pulls put via load - rated drawer slides. The back cushions become the rest of the mattress, and there are no legs holding up the extension. There's plenty of space to walk by when extended, and tons of storage underneath. We're really happy with the design and will definitely use the design again. We still decided to do a permanent bed for ourselves this time, and have not regretted it at all even with the space it takes. It has so much storage under it, too, since it's much higher than a sofa. Just some things to consider.
Good luck on your project!