I can't say that I have experienced a highlight changing in size while I'm dragging it.
It is true that if you click on the resize handle on the edge of the highlight while editing that it is possible to edit its size, but I very much appreciate that feature.
Regarding a highlight changing size while editing manual visibility, isn't this probably just due to the highlight being attached to a particular part of the synth which will get smaller as you move away from it and get larger as you get closer to it in the synth? I expect that is an intentional feature.
Perhaps you are using higher resolution touch screens and therefore the default size of a new highlight on such a screen is difficult to select and drag with something as large as a fingertip, but I haven't had any problems editing highlights' position/placement with a mouse and keyboard.
That doesn't mean that your complaint isn't valid for everyone using a high res touch screen, though.