16x20 prints of course need to be processed in trays larger than 16x20. That is the first easy and relatively inexpensive step. You will not need tongs, since you will be using your hands for all lifting and turning over of the print in the tray solutions. You will only be developing one print at a time - multiple prints and the additional handling of these large pieces of paper in the same tray are a recipe for disaster.
So how much solution goes into your developer, stop, and fix trays?
Easy - since you can develop 30 16x20 prints with a gallon of working dektol (1:2) developer -standardize your trays with a gallon of working solution - this will leave you at least an inch of developer in the tray bottoms, which are all you need to agitate and turn the individual print during developing, stop, and fix. Then carefully move your print to a wash tray for holding prior to hypo clear and toning. It is important not to process large prints in deep trays that have more than one inch of solution above the paper - the simple act of lifting and turning prints out of deeply filled trays is a major cause of large print problems. Remember to keep the solution to one inch in your large trays and you will reduce these simple hydraulic processing problems.