Hi im trying to install the PTR client and i have installed it before. But now when i have installed it and downloaded the client it does nothing. I got to the file of the PTR installer and there is a file called Blizzard updater. I double-click that but that bring me to an installation of 12.6 GB. I dont think the PTR is that big. Could somebody tell me what i am doin wrong. When I press PTR on the wow launcher it brings me to the PTR FAQ page.

If you check in this thread, or ask in it if necessary, then you may have more luck:
PTR - Can't enter the PTR? Discuss it here.

But yes if you do deviate from the steps in the installation sticky (which includes downloading necessary updates) then yes you might encounter issues accessing the PTR.