Try to find a copy of the book "Primitive Photography." It contains many ideas with general plans to build box cameras. I think you could easily adapt the plans to your specific purposes.

I built a 16x20 camera a few years ago. I bought a bellows off flea-bay which came from an old process camera.

You don't really need a bellows, however, if you go with a box type camera or a sliding box camera. You might also look up the HOBO camera, a box style 8x10 camera, for ideas. If you are not going to have a ground glass, you will be relying on zone focusing--knowing that the camera will focus at a set distance and using the lens' f-stop to bring areas in front of and behind that distance into acceptable focus. Just be aware that with portraits you will be working with extremely narrow depth of field, even stopped down.

As for the box size, the inside back of the box would of course have to be slightly larger than the plate, perhaps 17x21. That would give you enough room for the plate and some mechanism to hold the plate in place while you move the camera around. The depth of the box will depend on your lens. For landscape, I would choose a lens (I use both a 450 and 600mm with my camera), and then do the calculations of the lens' hyper-focal setting at a given f-stop to maximize sharpness for as much depth as you can. For the 450 mm lens, this would be slightly longer than 450 mm (the lens focuses at infinity at 450 mm, by pushing the lens out a little and stopping down some, you could increase the area of acceptable focus to bring more of the foreground into focus without causing the scene at infinity to go out of focus.) For landscapes, then, you would be looking at a camera 17x21x19 or 20. You could of course taper the length of the camera to make it lighter weight.

The same camera could probably be use for portraits just by swapping out the lens for a 300 or 370 mm lens. You would have to figure out how far from the camera the 300 mm lens focuses when pushed out to 450+ mm, but it may work out fine for close ups.

I assume you are talking about dry plates, as wet plates would have to be loaded and exposed while still wet.

One thing I would recommend is to do a quick and dirty mock-up of the camera using cardboard or matt board to test your design. Cardbpard is light tight, easy to work with and cheap.