This guide assumes that you have MediaPress & Paid Memberships Pro plugin installed & Active. Once you have activated this plugin, please visit Dashboard->Memberships->Membership Levels and click on add/edit membership.
Under the Other settings of the Membership level, you will see an option to specify storage limit for MediaPress for users having this membership level as shown below.
Specify the storage limit as you deem fit. You will need to repeat this for each of your membership level. That’s all.