Well, you can technically plate anything, but we recommend sticking to parts that are manageable. Don't bother with car bumpers or rims. They require huge tanks, huge amounts of power, and way too much time. Leave items that large to the plating shops.
The basic limitations are always going to be:
1. Can the part be totally immersed in the liquid contained in the kit? 1.5 Gallon kits come with 1.5 gallons of liquid. 4.5 gallon kits come with 4.5 gallons of liquid, etc. To test this, fill a container with an amount of water and see if the part can be immersed. You need at least 4-6 inches of space between the part and the sides of the tanks to get an even plate.
2. Will you have a big enough power supply? Nickel, Copper, Copy Chrome require 1 amp per 15 sq in of part surface area. Zinc/Copy Cad requires 1 amp/40 sq in. Chrome needs 1 amp/1 sq in. Calculate the area of the largest part you want to plate and then ensure you have an adequate power supply.