You can use those variables in any fields that allows you to type in text. For example, you can use it to name the file when saving it or you can use it to add document dimensions to a customized banner page.
In addition, you can use them in the job characteristics to check the dimensions of the document. You do not have to type them in the job characteristics you just sleet the variables from the list. The job characteristics are used for selecting workflows automatically on hot folders. They are also used for routing jobs in a workflow. In addition, you can use them to route in the print and finish components. Lastly, they are available in th distribute component.
However, to your specific question about checking the size of the job. That's available in the preflight component. You do not need to use these variables. It's one of the checks available in the UI.