Howdy!
The pre-patch is around 16.4 GB in size and will download over time from now till the patch is released. You can check if the pre-patch data has started to download by either checking the client’s overall size default is 70GBs or checking your World of Warcraft/Data/Data modification date.
Overall, we mainly suggest to focus on making sure the Blizzard app is setup to install future content:
Goto the Blizzard icon at the top left -> Select: Settings -> Game Install/Update -> Check" Apply Latest Updates and Download future patch data. Set Network Bandwidth for Future Patch data to 0 for unlimited, or leave at 100KB/s.
Thank you.