Do ovarian cysts vary in size?

Yes. Ovarian cysts do vary in size quite alot. In your age group, a small cyst is about 2-3 cm. I've seen cyst as large as 10 cm or more. Cyst less than 2.5 cm are probably functional follicle in your age group. A cyst may grow or persist over a few menstrual cycle or complete resolve.
Yes. Some ovarian cysts are quite large, like the size of a grapefruit. Others are very small and they can be any size in between.