Today I saw a notification in the Office 365 Portal about the correct sizing of Shared Mailboxes.
It seems that Microsoft is creating Shared and resource mailboxes with a max limit of 100GB. As the documentation states these mailboxes can only have a size of 50GB and when you need more space you need to assign an Exchange Plan 2 license to that mailbox to get 100GB of space for that mailbox.
This change of the size limit will be applied on July 30th, 2018. See this roadmap item for more details https://products.office.com/nl-NL/business/office-365-roadmap?filters=&featureid=31391
This triggered me to check all the shared mailboxes of one of my customers.
I created a small script to get all the sizes of shared and resource mailboxes and place it in a CSV file to check what mailbox needs a license