This command will show how to getting mailbox size in Exchange 2010 using PowerShell.

Mailbox Statistics

Get-MailboxStatistics [username]

For example;

Total Item Size

Get-MailboxStatistics [username] | ft DisplayName, TotalItemSize,
ItemCount

For example;

Storage Limit Status

To check either the user has reach a storage limit or not

Get-MailboxStatistics [username] | ft DisplayName, TotalItemSize,
ItemCount, StorageLimitStatus

Showing data for all user

Get-MailboxStatistics | ft DisplayName, TotalItemSize,
ItemCount, StorageLimitStatus