I typically buy 5 port, 4 way valves with 1/4" NPT input and output ports. The exhaust ports are generally 1/8" NPT. For the most part, I tend to buy off ebay again as can get them from China with shipping for about $10 - $12 each.
- Getting 1/4" NPT input and output ports will ensure the valve should be plenty large and flow air well for even larger air cylinders. 1/8" NPT ports are generally fine, but have run into small smaller valves that worked fine for smaller air cylinders, but too restrictive for bigger cylinders. This is defined by the valve's Cv value, and you can research that if you wish
- Need a 3 way valve? 4 way valves can be made into 3 way versions by plugging one of the output ports
- Need a 2 way valve? Less likely for our applications, unless making an "ankle tickler" or an "air cannon". If making an air cannon, you will likely be using a sprinkler valve to flow a lot more air, or looking at a valve with 1/2" or larger ports