This is similar to the post on finding mails older than specified days in Outlook 2013 and Outlook 2010. Let us see how to search emails based on their size in Outlook 2010.
Click File menu, navigate to Info menu and click the drop down arrow available below the Cleanup Tools.
From the Cleanup Tools drop down list, select Mailbox Cleanup.
In the Mailbox Cleanup window, navigate to Find section.
Using the Find items larger than radio option, you can specify the size of the emails that needs to be searched. By this way you can do a find based on the email size.