Did you create a new FTP user when you set up this Fully Hosted Domain? With that, they can upload whatever they want (pages, WordPress, etc.) in their own space.
You would still have to set up any databases they’d need. I recommend that you set up a few just so they’re available. Give them generic names, like DOMAIN_db1 with a user DOMAIN_user1, etc.
Account Privileges will give them some access, but you’ll still need to assist them once in a while.
There isn’t a way to restrict the disk space they can use. It’s “unlimited,” after all. Just keep an eye on it and email them if it gets out of hand.