In principle, whatever the disk size of your mServer is the amount of additional backup space you will need. So, if your disk size is 50GB, you will need an additional 50GB of backup space to store the backup copy of your server.
In case you intend to use the auto-backup option (recommended) you also need to take into account that the system will create 2 different copies of your mServer (daily and weekly). These copies will be automatically replaced daily and weekly respectively.
Because of the way the backup process works, you will also need to include space for an additional backup copy (the system creates a new backup copy first and then deletes the old one).
Hence, you will need to purchase enough space for storing three copies of full size disks. For example, if you are currently using a 50 GB disk, you will need to purchase 150GB of backup space.