Keycaps are unit size. Alphanumerics are 1 unit or 1x1. Larger key will be marked like 2.25 it is 2 and a quarter size of alpha keys. The keys on the bottom like Control and etc would be 1.25, \| is 1.5. If you're buying a full set all those are included, so nothing additional is required unless you want to get additional different color for something.
WASD or those cheap sets at EK are basic quality like the stock keys that came with the keyboard. If you want to really improve experience I would recommend some Cherry doubleshot or PBT dyesubs key set. You can just go to the top of quality and be done. Have a look at WTS in the classifieds as people are often selling lightly used sets, or at originativeco.com. If you're not in a hurry there is the groupbuy section also.