Limit the harddrive storage of multiple WordPress instances on one webserver

You will need to make the restrictions on a web server level, not within WordPress itself.

You can restrict table size in the database or restrict folder sizes on the server itself.

Doing this however will break your site. If you restrict a folder to be no more than “X”, then it can’t process the PHP to render the page as there’s no disk space available. Similarly, if you restrict a table or database size (most web hosts restrict their MySQL databases to 1GB by default), there is no way to generate temporary sessions etc.

Check with your web host on the best-recommended practice for your particular environment – just be aware, the moment you put a restriction in place, you will break your site if you ever hit that limit.