This article assumes you have already created a template. If you've yet to create a template you should do so first.
Firstly, edit your template by finding it in the template listing and clicking the Edit button. Upon clicking this button you will be directed to the Template Builder where you will be able to update your template's width, height, and margins.
If the Properties Inspector is not already open, open it by clicking on the bar on the right.
Once the pane opens, ensure that the Template option is selected at the top of the pane. From the displayed options, you will be able to change the width, height and margins of the template in millimeters.