Ok, I think I found a way to do it. Basically, in app->res->value right-click your mouse, and select: ->New->Values resource file.
Filename is whatever you want (I used "dimen") as long as you refer by the same name later, and click "OK". That file is created and you can store your dimensions inside "resources" pair of tags.
The format is the following:
<dimen name="dimension_name">dimension</dimen>
where dimension_name and dimension are parameters set by you.
Later on, to use them in your XML you type @your_file_name/dimension_name
where your_file_name and dimension_name were chosen by you, earlier.