Increase or customise EXO mailbox size.
You should use Exchange Online PowerShell to do this.
Let's increase the mailbox size from 3 GB to 10 GB.
Execute your PowerShell ISE as a administrative permission.
And execute this code.
Once you execute this, a little screen will pop-up there and log in with the global administrator account in your tenant.
Install-Module ExchangeOnlineManagement
Connect-ExchangeOnline
Get-Mailbox testuser02@m365x51911478.onmicrosoft.com | Select *quota*
The result will be like this.
Ensure that the UPN and execute this code.
Set-Mailbox testuser02@m365x51911478.onmicrosoft.com -ProhibitSendQuota 10GB -ProhibitSendReceiveQuota 10GB -IssueWarningQuota 2GB
and check agian with this code.
Get-Mailbox testuser02@m365x51911478.onmicrosoft.com | Select *quota*
The results:
You might be want to change the mailbox size of the specific group member.
In this case, you need to identify your group type and select the code you need to execute. There are Unified group and Distribution group in the tenant.
Get-UnifiedGroupLinks -Identity "tester" -LinkType Members | Set-Mailbox -ProhibitSendQuota 10GB -ProhibitSendReceiveQuota 10GB -IssueWarningQuota 2GB
Get-DistributionGroupMember -Identity "tester2" | Set-Mailbox -ProhibitSendQuota 10GB -ProhibitSendReceiveQuota 10GB -IssueWarningQuota 2GB