Hmm, I would give you a stainless steel maison jar of beans of a standard size. Once you're done with the beans, I'll come collect the jar. The same goes for messy foods.
Short answer is "I don't know" unfortunately. I'm working on figuring all of that out. I would assume there'd be fixed sizing. For the returns, I will collect the containers from the front door or a bin at the front desk and sanitize them at a commercial kitchen and return them to the food producer.
I have not figured out how much people want to pay yet; I'm interested in making deliveries and creating a super loyal customer base first, however small. Definitely gotta start thinking about all you've mentioned.