As you can see the + (Custom) sign at the end of the screenshot. You can always define a custom preset from here . Just set the required dimension in the dialog box:
You can also define these in document presets i.e. File - Document Preset - Define and you can define several custom presets. I think you don't have any option to delete the current default available presets.
Thank you for the answer, rohit. I've already tried your solution, and although it does provide me with the possibility of creating presets, it does not appear in the presets window, that I have linked to in my original post - that is what I am looking for a solution for...
Click on the + in the circle.
I'm sorry, I may not be explaining this properly... What I wish is to do is be able to replace the current presets (ie. from the screenshot in the first post: "Surface Pro 3, 2160x1440 px, RGB") with a preset of my choosing.
For example, I just clicked on the icon with the + in a circle, created a document with specific measurements that was not available in any of the current presets on the page - despite of this, it does not appear as a possibility in the presets-window, when I go back.