one of the first things that I found confusing is the fact that it doesn not yet seem to be possible to resize layers or widgets using the mouse, instead the dimensions need apparently to be entered manually?
Layers can not be resized using the Mouse, but it is possible with the widgets. You need to press the uppercase (arrow) key to do it while using the Mouse, because by default the mouse moves the widgets.
confirmed, this is working like you said. Probably, this would be another good "hint" to be displayed in the "log" area then. Sort of like "tip of the day", without being obstrusive.
Alternatively, one could change the behavior to add clipping detection, so that resizing would only work for mouse movements while clicking on one of the widget borders (pink lines)