The individual mailbox size limit is based on the assigned toolkit. Consult this article to find out which toolkit was assigned to you and the mailbox size associated with that toolkit: Microsoft 365 Licensing – Toolkit overview and eligibility.
Shared mailboxes have 50 GB of Storage.
To find out how much mailbox storage you are using, follow these instructions:
After your mailbox reaches its full capacity, you won't be able to send or receive messages. The solution is to reduce the storage.
Note: If you have emails you would like to archive, do that first before emptying the folder.
After that you can empty folders to free up Storage by following these instructions: