Q: What size kit do I need?
A: The plating tank needs to be large enough to fit your parts, with a few inches space around the part to spare. You also need enough plating solution to immerse the part totally. Here's an example of how to work this out:
Part is 6" x 6" flat piece of metal. To fit the part, the tank would need to be at least 8" diameter or wide. If you use a rectangular tank, a tank 8"L x 6"W x 8"H would suffice. The volume of that tank would be 384 cubic inches (8*6*8), or 1.6 gallons. So a 1.5 gallon kit would probably be fine.
Part is 12"x12"x12" cube shaped. To fit the part, the tank would need to be 15"L x 15"L x 15"H. That tank is 3375 cubic inches (15*15*15) or 14.6 gallons. You would need a 15 gallon kit to plate this part.