The Dimensions tab allows you to set the default size for the given dimensions. To edit a size, just use the primary mouse button to click the size you want to edit, and change that value.
When you are finished making changes, select OK. If you do not want the changes that were made to be saved, select CANCEL.
Note: The drop-down list under Manage Levels By can be used to select a Level Type to use when managing options (instead of a Section).