Ideal pressure cooker size for cooking a pound (454g) of beans

I've generally had success cooking dried beans in a slow cooker for about 5 hours on high with plenty of water (about 4 c. per 1 lb of beans). While you COULD add everything at once, I wouldn't recommend it, as the rest of your vegetables will become mushy and grainy.

For that exact recipe, I'd cook the beans in the slow cooker, than combine everything on the stovetop for about 45 minutes to get the flavors to combine.

There is a complicated formula for calculating cooking time based on energy input, surface area, thermal conductivity of food etc. Smaller pieces with a larger surface area will cook slightly quicker than large single pieces

In general for roasts (large slabs) just add the weight of the pieces and cook for the minimum time recommend by your pressure cooker for that (or from your experience). Then test using an accurate thermometer and cook some more as required. In my experience most people overcook meat anyway, so try it a little more undercooked for a change. Remember to let it stand at least 5 minutes per 500 g too

There are too many variables to give a blanket answer (stove energy, pressure in pressure cooker, meat type, fat content etc.)