So, disclaimer: I’m a newb myself. That said; I couldn’t really find any tutorials on the kind of moving and scaling “window” widget interface I wanted to do for my first project. Now that I have something working, I thought I would contribute. Give it a gander.
https://…com/2015/09/25/tutorial-movescale-window-widgets/
https://www…com/?v=e-c-86Jh8Uk
Hope it helps!
Not tried replicate it yet but on reading i discovered where is the problem with my resizable widget (or i think is it), i not used canvas panel as root/top layer, i think this is the cause of why i can’t access to the slot of my “minimap” placeholder widget (and i need it to keep track of minimap size).
For who not understand what i said, i’m talking about this
Tomorrow i will start replicate your tutorial and go deep on it.
Meanwhile thank for sharing 
For who interested i replicated the tutorial, you can download the project here.
Blueprints are commented with the same contents of the blog, the only difference is i used a custom player controller instead of a character blueprint to place key bindings.
Much appreciated, .