Cescolite Heavy-Weight Plastic Developing Tray (White) - 16x20"

this tray is flat bottomed

The outer dimensions are 20 x 24 x 3.

The first problem with using the minimum amount of solution for processing prints is that it will likely exhaust the developer and not have enough to do any given number of prints above one or two. If you do any test prints they'll use up all the reactive developer before any final prints make it in, so finding an exact number would be an educated guess at best. The surface area of a 16 x 20 in tray is 320 square inches, so if you filled it half an inch deep you'd have 160 cubic inches of fluid. Those 160 cubic inches converts to 0.7 gallons (0.692641 gallons exactly) or 2.6 liters (2.6219314035 liters exactly). It won't be the minimum, but it's simple enough math to outline a potential way of determining it. Or the best I could do by hand.

There is no easy way to do this. You do not want to scratch or pit the plastic surface. A weak solution of Clorox and water works well for general cleaning. Most darkroom workers label their trays and dedicate them to one purpose. This is less important for stop bath and fixer, but I do believe it is important for developers. Contamination is to be avoided at all costs.