What size pressure tank and switch should I use on a submersible well pump with approx. 12 gpm flow? This is a recently purchased home that had the tank, switch and piping removed.

This seems to be a pretty good site to answer your question. The two variables you will have to decide on are: 1. Cycle length (how long the pump should run to refill the tank). 2. Pressure cutin/cutout points for the switch.

How to Size a Pressure Tank

I entered a two minute cycle and a 30/50 switch and it came back as a 77 gallon or so tank, which sounds reasonable for a 12 gpm pump. It's probably easier to go too small than to go too large. You don't want to buy a tank so small that the pump only runs a minute or so to get pressure back to 50#. Two minute cycles or longer are better for the pump.

So far as a switch is concerned, try calling the folks here. They will likely sell you a square-d pressure switch pre-set for 30/50.

Well Water Tank Pressure Switch - FreshWaterSystems.com