What is the difference between a follicle and a ovarian cyst? Can a follicle grow to a golf ball size?

Size. Follicles come and go normally in menstruating women. Cysts are just somewhat bigger follicles, around 2 cm. Follicles can sometimes develop into large cysts. A golf ball size cyst would not be uncommon. In any event these are not cancerous and usually resolve after a couple of menstrual cycles.
A kind of cyst. There are many kinds of cysts. A follicle can become a follicular cyst or a luteal cyst. An ovarian follicle is what matures and ruptures to release an ovary. Sometimes it doesn't rupture and just stays closed off as a follicular cysts, which can grow up to 2cm in size. After it ruptures, it can also seal off and form a luteal cyst, which is eventually absorbed.