If you have a provider that allows overselling than you can set up plans that have more space and bandwidth than your total plan allows. This is based on the theory that your client will not use the space and bandwidth given. If you have a reseller account that does not allow overselling, then you have no way to make the plans any larger than your total allowed.
But, yes, that is it 20GB or whatever the plan you choose in total, when the space is used you are out and would have to upgrade.
If you are interested in reselling I would recommend a dedicated server which will allow you much more control. They are very inexpensive these days
