You can find out in the Control Panel; search for "index" or so, and you should find it quickly. Change what it indexes (e.g. just your "favorites" and the start menu), and rebuild the index, under "Advanced".
On my system, disabling it for most of the system took the index's size down from ~23.3 GiB to less than 80 MiB, which is a significant amount of space even today, especially if you're running off an SSD.
As usual, I recommend using WinDirStat to find out what is ACTUALLY wasting space on your drive(s). Be sure to run it as an Administrator, or it won't have access to various directories (including, incidentally, the one containing Windows's search index).