Hello! First, thanks for the Plugin.

I am trying to reduce the size of the three header boxes (Character face, menu and gold and items.
Could you tell me in which part of the plugin do I find these dimensions?
My idea is to use backgrounds (bgimg) to provide information about skills.

We are using MZ and are having similar issues. Based on reading previous posts, the images in the background of the boxes are not yet supported in MZ version. 

I can help you with the size of the dimensions, there is a post lower down in the community section here where Felski describes how to change the node sizes. We were able to do that.