For reasonably priced stockings, it's hard to beat Polish brand Fiore. Every season they release exciting new lines with complicated woven patterns, a range of deniers and finishes, and flat packaging (which you will appreciate the next time the postal service try to crush your parcel!).
Hosiery sizing is always what we call volumetric - that is, in an ideal world it uses both height and size, because it's stretchy in all directions. Broadly speaking though, we'd break down the sizes by height:
Size 2 (Small) - 5'2-5'4, 152-163cm
Size 3 (Medium) - 5'4-5'7, 162-170cm
Size 4 (Large) - 5'7- 5'10, 170-178cm
If you know that you have a curvier or larger leg than average for your height, go up a size, or if you have a narrower or more slender than average for your height, go down a size. Generally because Fiore products contain a large percentage of lycra, all that will happen if the stockings are too small for you is that they will end up running short. Wearing a size to large will result in a less sheer appearance.