I see that you clearly stated your looking for a solution in SSMS but I thought I would provide a PowerShell solution in case it helps. (SQLPS is accessible from inside of SSMS 2008 & SSMS 2008R2)
You can use SQLPS (or regular PowerShell with the SQL cmdlet snapin) to run something like this:
Invoke-Sqlcmd -Query "sp_databases" -ServerInstance "LocalHost\R2" | Export-Csv -Path "C:\temp\MyDatabaseSizes.csv" -NoTypeInformation
I can keep going with this example if you're interested.