Blackwidow using DSC(a height standard for keycaps) profile keycaps which is similar to most other modern mechanical keyboards out there such as FILCO and DUCKY.
Though, the stock keycaps on BW is somehow a tad taller than most other OEM DSC keycaps.
If you want to change the keycaps, the whole bottom rows(Ctrl, Alt, Spacebar and so on) cannot be used, you can use your eyes and brains to judge by comparing it's dimension with other keyboards(simply google for a normal mechanical keyboard).
In case you want the exact numbers, most modern keyboards using the standard of (starting from left Ctrl)1.25x, 1.25x, 1.25x, 6.25x, 1.25x, 1.25x, 1.25x and 1.25x sized for the bottom(Ctrl row), and the total of them are 15x. With the same total length(15x), Razer BWU(or I should say iOne) go for 1.5x, 1x, 1.5x, 6x, 1.5x, 1x, 1x and 1.5x.
If you are getting blank keycaps, there are 2 possibilities. One is that there will be a mark under the keycaps telling you the height of the particular keycaps(R1, R2, R3 and so on). If none, then you need to arrange them together, comparing them and seperating them.
For removing the keycaps, you can buy optional keycaps puller from WASD.