Unless I'm mistaken, plesk is a front-end that might be supplied to you by your hosting provider. I seriously doubt you will be able to change this value yourself if you are using a hosting provider. If you ARE using some hosting provider to host your site, ask them about changing the setting. I'm not certain, but I believe it's in the mysql INI file, meaning that someone will have to edit the INI file and then restart the mysql service.
Ask your hosting provider. If you have complete control over your server (i.e., root-level access) then you should check out the installation guide:
http://dev.mysql.com/doc/refman/5.1/en/installing.html