Run this SQL command to delete all revisions: DELETE FROM wp_posts WHERE post_type = "revision"; Code (markup): Put this code into your file wp-config.php in order to prevent revisions from being created in the future: define('WP_POST_REVISIONS', false); Code (markup):
Better to use plugin for removal instead of sql query. http://wordpress.org/extend/plugins/revision-removal/
I prefer doing the devtard method as it is easy and require no additional plugins (I believe less plugins is better). However, using plugin is recommended as it hassle free, has lots of features and almost everyone can do it. For OP. deleting revision is good to keep database smaller and faster to access but i recommend to delete only old revision and allow new revision to be created. Or better, only keep future revision but limit it it just one. Deleting spam comment also decrease the database size. Choose whatever the best for you and it would be great if you keep inform us on what you have done to solve your problem. Arick.
Also, once you remove all the revisions, in your wp-config add the line below. It allows you to set a total number of revisions you want to keep, so you can still use the functionality without overloading your database: define('WP_POST_REVISIONS', 3);
Get a plugin like WP-Optimize. It will remove revisions, auto-drafts, clear SPAM and keep your database tables optimized.
Devtard was right with his advice if you plan to delete the existing ones and turn them off (which helps a lot when your website gets bigger), samir's code limits revisions to 3.