This is not Adobe. Rather essentially a user to user forum.
And, this is Premiere Elements and not Photoshop CC.
It has already been brought to the attention of Adobe that the bounding box representation in 14/14.1 is not like previous version of Premiere Elements..
But, please consider filing an Adobe Feature Request Bug Report Form to make your Premiere Elements thoughts on this known to Adobe.
Do you also have Photoshop Elements 14? If so, what are your comments on that? I would be interested in knowing of your comparison of the scaling in that photo edit versus scaling in the 14 video editor.
You make reference to guess work in % scaling in the 14 situation where the bounding box does not ever visually exceed the limit of the monitor space. With the clip highlighted/selected, can you not go to Applied Effects Tab/Applied Effects Palette/Motion Panel expanded and read the % scale if the visual determination of the scale effect at the TImeline level does not suffice?
Best wishes for the new year.
Thanks for the comments. I can navigate as you suggest but all the sizing action happens in the first millimetre of the scale! it's impossible to adjust without lurching from small to massive and back!