Choose a directory on your disk, it will contain all the files needed for your wiki instance. At the beginning, your wiki instance will use approximately 10 MB of disk space. Then of course, it will grow depending on the way your wiki is used. A personal wiki, even with many pages, might only use 30 MB or 40 MB of disk space. A popular wiki, or a wiki with many files attached to the pages, might use much more, of course.
If you are the administrator (or root) of the server, you can use anything you like or that makes sense to you, for example /usr/local/var/moin, /mnt/wikis, etc.
If you are a simple user, you will probably only be allowed to write in your personal, "home" directory. Choose a subdirectory that makes sense to you, for example the share/moin subdirectory.