Short version:

Public Sub ShowQuotas()    Dim oStore As Store
    Dim propertyAccessor As Outlook.propertyAccessor
    
    For Each oStore In Outlook.Application.Session.Stores
        Set propertyAccessor = oStore.propertyAccessor
        If oStore.ExchangeStoreType = olExchangePublicFolder Or oStore.ExchangeStoreType = olPrimaryExchangeMailbox Then
            PR_MESSAGE_SIZE_EXTENDED = propertyAccessor.GetProperty("http://schemas.microsoft.com/mapi/proptag/0x0E080014") / 1024
            PR_MESSAGE_SIZE_EXTENDED = PR_MESSAGE_SIZE_EXTENDED / 1024
            Debug.Print "PR_MESSAGE_SIZE_EXTENDED (Inbox size): " & Round(PR_MESSAGE_SIZE_EXTENDED) & " MB (=" & Round(100 * PR_MESSAGE_SIZE_EXTENDED / PR_QUOTA_RECEIVE) & "%)"
        End If
        Debug.Print "------------"
    Next
    Set oStore = Nothing
End Sub