Get Sum of Exchange Mailbox Statistics

I'm trying to get the sum of the TotalItemSize for an Exchange database using the command

Get-MailboxStatistics -Database MBX07 | Measure-Object -Sum TotalItemSize

The command works perfectly fine in the Windows PowerShell ISE but if I run it in an Exchange EMS (both are on my local machine) I get errors for every mailbox in the database that say

Measure-Object : Input object "8.518 MB (8,932,049 bytes)" is not numeric.

The output in the ISE where the command works looks like

Count    : 174
Average  : 
Sum      : 203481256406
Maximum  : 
Minimum  : 
Property : TotalItemSize

It's an Exchange 2010 SP1 server running on Windows Server 2008 R2 and I'm running Windows 8.1 64bit

Any help is greatly appreciated

This was run from my EMC on my server.

Get-MailboxStatistics -Database MBX07 | ForEach-Object {[Microsoft.Exchange.Data.ByteQuantifiedSize]::Parse($_.TotalItemSize)} | Measure-Object -sum

TotalItemSize is of type Microsoft.Exchange.Data.ByteQuantifiedSize so we use its method Parse to get a value we can put into -sum

More on this here

BONUS

You can try this which will output the Sum in MB

Get-MailboxStatistics -Database MBX07 | ForEach-Object {
     ([Microsoft.Exchange.Data.ByteQuantifiedSize]::Parse($_.TotalItemSize)).ToMb()
     } | Measure-Object -sum