<a href=”http://technet.microsoft.com/en-us/library/aa996177.aspx”>Public Folder Store Database Size Limit</a>
Hope this helps!
The above answer shows you how to limit the database size for public folders. However if you just wanted to limit the public folder itself you can create a policy for this.
1. Open Exchange System Manager
2. Expand System Policies
3. Right click and create a Public store policy.
You can then even go deeper than that and when you create folders within the main Public folder you can then go into the properties of the folder and choose limits there as well.