GDrive Storage is to be ordered per account. It can be changed later from one account to another. You can not share the selected sizes, the size options that are available to choose from, can only be set per account.
Every account have 30 GB as storage as standard. Selected sizes below is an additional storage. For example, if you select 20GB the account gets 30+20=50GB. If the account storage need grow even more, you have to change the existing extra storage plan the account have.
If you need, for example, additional 50 GB to every account of total 5 accounts, fill in 50GB, write 5 accounts, then specify under "Other" which accounts this regards. Do you need different sizes of different accounts, just specify this under "Other".
Contact:
Avalon Order Management
ordermanagement@avalonsolutions.com