Keycaps are measured in "u's". your standard alpha key is going to be 1u. A shift is often 2.5-3u. Space bars can be 5.5-7u depending on the baord.
If you can't find a layout in the documents for your board you can simply measure the "standard" cap and use that as your reference to figure out the rest of your caps.
Keep an eye out for weird things like your ctrl, alt, fn being different sizes on the left and right side.
Most good Keycap sets will either include a whole bunch of alternative sizes or have sets you can purchase on top of the base set to fill out your missing pieces.
Have fun!