To limit the bandwidth for a website from Plesk 10,please follow the following steps  :-

1) Login to your Plesk control panel.

2) Click on "Domains" and then Click on "Control panel" in front of the domain whose files permession you want to change.

3) Click on "Websites and Domains".

4) Click on " Show Advanced Operations ".

5) Select "Switch on bandwidth limiting" checkbox.

6) In the Maximum bandwidth usage (KB/S) box, specify the maximum speed, measured in kilobytes per second, that a
website can share among all its connections.

7) Select  "Switch on connections limiting" checkbox to limit the number of connection.

8)  In the Connections limited to box, specify the maximum number of simultaneous connections to the site.

9) Click OK.