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I see that you clearly stated you're 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 & 2008 R2).

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.