Using older versions of Outlook for Mac in business environments
There is no setting in Outlook Mac. Sorry. If this is a feature that you consider useful, be sure to let Microsoft know by sending them feedback (via the Send Feedback command in Outlook’s Help menu)
http://www.microsoft.com/mac/product-feedback
For more differences between Outlook Mac and Outlook for Windowssee this link.
Are you sure you aren't over the attachment limit set by your ISP?
If you are using Exchange, Exchange 2010 comes set to default 10MB. Your IT will need to change this.
Limits
Hotmail : 10MB
Yahoo : 20MB
Gmail : 20MB
See this article for sending limits from several ISPs.
http://www.emailaddressmanager.com/tips/email-rate-limit.html
You send it "Lite". Up to 100MB free.