Some of my customers mailboxes were still displaying 25gb as their mailbox storage quota size instead of 50gb.
So the fix is quite simple.
Firstly connect to Exchange Online via Powershell.
1. Set-ExecutionPolicy unrestricted
2. $LiveCred = Get-Credential
3. $Session = New-PSSession -ConfigurationName Microsoft.Exchange -ConnectionUri https://ps.outlook.com/powershell/ -Credential $LiveCred -Authentication Basic -AllowRedirection
4. Import-PSSession $Session
Then run the following CMDlet which will increase the size to 50gb for all user mailboxes:
Get-mailbox -resultsize unlimited | Set-Mailbox -ProhibitSendReceiveQuota 50GB -ProhibitSendQuota 49.75GB -IssueWarningQuota 49.5GB