Use case or problem
As of v1.12, drag-resizing images is possible via the live preview editing mode. Personally, though, I much prefer using the source edit mode, and therefore (to my knowledge) am stuck with manually typing sizes, which is a lot more effort to test and tweak than dragging to resize would be.
Proposed solution
Source edit mode (rightfully) displays images as embed links, so it only makes sense that reading view gets the option to resize images via dragging instead, very much like how tables can be resized in reading view but not source edit mode.