Get-MailboxSizeInMB.ps1

#requires -Version 2
#requires -PSSnapin Microsoft.Exchange.Management.PowerShell.SnapIn
$xDays = 90

Get-Mailbox -ResultSize Unlimited -RecipientTypeDetails UserMailbox |
ForEach-Object -Process {
    $MailboxStats = Get-MailboxStatistics -Identity $_.DistinguishedName  

    $si = Get-MailboxFolderStatistics -Identity $_.DistinguishedName -IncludeOldestAndNewestItems -FolderScope SentItems 

    if($si.NewestItemReceivedDate -AND (New-TimeSpan -Start $si.NewestItemReceivedDate.ToLocalTime()).Days -ge $xDays) {
        
        New-Object -TypeName PSObject -Property @{
            Name               = $_.Name
            Alias              = $_.Alias
            PrimarySmtpAddress = $_.PrimarySmtpAddress
            WhenCreated        = $_.WhenCreated
            Database           = $_.Database
            LastSentItemDate   = $si.NewestItemReceivedDate
            LastLogonTime      = $si.LastLogonTime
            MailboxSize        = $MailboxStats.totalitemsize.Value.ToMB()
        }
    }
} |
Select-Object -Property Name, Alias, PrimarySmtpAddress, LastSentItemDate, WhenCreated, LastLogonTime, Database |
Sort-Object -Property LastSentItemDate |
Export-Csv -Path C:\temp\Inactive90Days.csv -NoTypeInformation -UseCulture