Suggestion: Current Tag Panel Field sizes are Large (full width), Medium (1/2 width), Small (1/3 width) and Smaller (1/4 width). Suggest adding 2/3 and 3/4 so that Tag Panel can be utilized better with fewer rows.
I was just about to suggest the very same thing myself; although I was going to propose only adding 3/4 because this would be easier to add to the current naming scheme and it would result in a symmetrical number of larger and smaller sizes (i.e. 2 larger than medium and 2 smaller than medium):
However, seeing as there is already a 1/3 size panel width option provided, then it would make sense to have a 2/3 size too, otherwise you are limited to what other panel sizes can share the same line as a 1/3 width panel.
I'm not sure what the best naming for 7 panel sizes would be though. Perhaps the most logical and intuitive labeling would be to add a category above the 'larger' size, which has a name that indicates it is not fractional like the other categories (for example: Maximum, or Full Width).
This would correspond to:
I've just released Mp3tag v3.19a and added the Tag Panel field sizes Full, Larger, Large, Medium, Small, Smaller, and Tiny.
Thanks for the initial idea @BobDrum and the detailed suggestions @Quisquose.