The manage sizes module is universal across CobaltConnect so CobaltConnect users cannot add sizes on their own. To add a new size select the support ticket and add the new size you need. It is not guaranteed that it will be added.
Changes to a product's size updates immediately into the My Products module.
You cannot delete a size from the Manage Sizes module, but having a size in the module will have no effect on your products unless products are associated with it.