The atria can undergo dilation in response to chronic volume overload. Conditions that increase volume overload are... mitral valve regurgitation, the volume and pressure of the left atrium are greatly increased. The left atrium responds by undergoing chronic dilation, which enables it to accommodate the increased volume without as large an increase in pressure because of its increased compliance.
Dilation of the atria are results of cardiac remodeling. Under some conditions ( exercise training) remodeling is beneficial; however, under other conditions ( heart failure) this remodeling is detrimental because it increases the oxygen demand of the heart and decreases mechanical efficiency. Certain drugs, such as beta-blockers, ACE Inhibitors, and angiotensin receptor blockers have been shown to prevent or partially reverse remodeling under pathologic conditions.
Atrial Fibrillation/Flutter can also cause romodeling in the atria.
Other than your current diagnosis, do you have any other cardiac history?
Here's something I found on the web, you'll have to copy or cut and past in your browser, this side will not hyperlink to external sites.
http://en.allexperts.com/q/Heart-Cardiology-964/2008/7/Echocardiogram-biatrial-dilation-normal.htm