The best way to start is to actually measure some of the things you are used to around the house. If you sleep in a single bed, measure it. It will surprise you how big beds actually are. If you think you can sleep on a smaller one, try it, put some stuff on your bed to make it smaller and sleep in it for a night. If you can't handle the smaller bed comfortably, then you need to allow for a larger bed. In your first drawing, the bed is 18" wide. I would hang over both sides, literally! A regular couch is wider than 18". Make a list of what you need in the bus. If you need a fridge, figure out how large of a space you need. If you need a table, actually sit at a table and measure how much room you need to be able to sit down and stand up, it is hard to get up with a chair pushed in all the way!
I went through the same thing. My scale was all out to lunch. Once I realized how much room things take, I knew I wasn't going to get as much in my bus as I wanted. BTW my bus is a 28 ft conventional, bumber to bumper, and has 19 ft of space from the back of the drivers seat to the back wall inside. Almost all buses are 7'6" wide inside, and a little over 6' high in the very center. Good luck with the conversion, and once you find your bus, or should I say, once your bus finds you, we need lots of pics.