You need an external drive to store the image while you erase the drive, if you do not have one stop now, get an external drive and continue.
All 3 of the sizes you chose are small, 2 of them are far too small.
Windows XP can run in 32GB
Windows 7, 60GB is a workable minimum, 100G is far more sensible. 32GB is a joke and less than that simply won't work.
Windows 8, same as 7
If installing XP (next time tell us which version) you will need the original installation discs for your Mac.
I have an external hard drive where I can put the image of the disk while erasing it.
Since I have 53 Go free on my Mac Disk, my windows partition can be up to 40 Go.
When I have made that copy, what do I do with the Disk Utility ?
I just don't understand why I have to completely erase the disk in order to partition it ? Anyway, I want too much to play my old PC games, so I will do it