This has been ask so many time and @Jesse answer it many times too but searched but doesn’t seem to have tutorial about this.
So, here it is quick video how to make Entity Screen to fullscreen (not exactly fullscreen but fill all the space).
Thanks to Kendash for the tip.
Steps:
Enable Design Mode
Select Widget
Right Click and select Property
Set X and Y to 0
Adjust Width to maximum (your screen resolution width mine is 800x1280.)
Adjust Height until you get fullscreen.
While playing with this, I noticed that sometime widget won’t save. I have to delete the widget and add it again. It seem it only save once.