The mailbox type and the user's subscription license determine the amount of mailbox storage available. Administrators can reduce maximum mailbox sizes per user or globally.
To view Quota of mailbox, you can connect to Exchange Online PowerShell and run the below cmdlet
Get-Mailbox JDoe@contoso.com | Select *quota*
To Set Quota of mailbox, you can connect to Exchange Online PowerShell and run the below cmdlet
Set-Mailbox JDoe@contoso.com -ProhibitSendQuota 19GB -ProhibitSendReceiveQuota 20GB -IssueWarningQuota 18GB
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