the most common sizes of these cylinders are stock stroke, 4mm and 10mm. they come in 65mm or 68mm bore.
most likely you would purchase from a builder who would tell you everything you need. all stock parts fit the cub cylinders, however, for maximum performance gains bigger carbs, an aftermarket head, intakes and reeds are recommended.
depends on what kind of riding you do. cubs are mainly a drag only cylinder. they have a light switch powerband that comes on about 6,000 rpm. they are not good for lugging through the trails or play riding. some people dune with them and love it. i personally use my 10mm cub bike for hillclimbing and drag only.