To check a shared mailboxes size in PowerShell, you can use the below cmdlet. Copy and paste the command in PowerShell
#To check Shared mailbox size
$SMbx = Get-Mailbox -RecipientTypeDetails SharedMailbox -ResultSize Unlimited
$Report = @()
ForEach ($S in $SMbx) {
$Stat = (Get-MailboxStatistics -Identity $S.Alias | Select ItemCount, TotalItemSize)
$ReportLine = [PSCustomObject][Ordered]@{
Mailbox = $S.DisplayName
TotalItems = $Stat.ItemCount
MailboxSize = $Stat.TotalItemSize
Quota = $S.ProhibitSendReceiveQuota
Licensed = $S.SkuAssigned}
$Report += $ReportLine }
$Report | Format-Table Mailbox, TotalItems, MailboxSize, Quota, Licensed -AutoSize