This reddit post talks about the download locations of the patches: https://www.reddit.com/r/ffxiv/comme...o_the_c_drive/

In short, in your My Documents folder, there is a folder call My Games. That is where the files download first before the patches are applied. If you want, you can change which drive the My Documents folder is and then you do not have to worry about the space on the C drive.

Edit: I wanted to let you know that if you do change it, be aware that some applications use that reference for files too just like FF14 does in this case.