I love the 5 sizes: Multi-line, Large, Medium, Small, and smaller, but I would love a smallest field that will at least get you 5 (preferably 6) fields per line.
Sometimes you just need 6 or less characters to populate what you want on a field. It would be great as far as an economy of display real estate to have smaller fields so one can fit more info into the tag panel.
I agree that it would be nice to have even smaller fields possible with even only 1 character to show. But this is not really practical because you need a speaking field label and that mostly needs more characters.
+1 
Typical real examples for "smallest" field size would be YEAR, DISCNUMBER, TRACK for example:
As you can see it would be enough width if 4 or 5 letters like Year or Track could be placed as description label over such a field.
Rating, Dynamic Range, disc number, track number, movement number, year, release, there are so many that can be even 1 or 2 characters. In fact there could be an argument for 2 smaller sizes: smallest and mini, the later being for fields that require 3 or less characters, and the former being for 6-8 or less characters.
It would also be great if the album cover display size was not solely predicated on the width of your vertical panel, as it seems to be.
With screen real estate so important this would be a great improvement, at least in my workflow. I am glad others agree. Maybe this will get some traction.
I think like poster touched on at a certain width the label of the field would be longer than the field itself, such as 'Publisher', 'Total Discs', etc.
Just tested how the window reacts to four of the current shortest fields in one row and any two-word (or more) label names truncate to just the first word. So 'Album Gain' becomes 'Album' (no ellipsis).
For users who really want an even shorter field I suppose they wouldn't care since they'd know what the field is from familiarity but just something to consider for the UI.
There are those GIFs! Yep, I would not. But as I read in another thread about this subject, this is where further customization would really be the way to go. There are several programs I use, foobar being one of them, that allow you to customize UI to your heart's content, like changing character size, field size, separators, shading etc, but I doubt that is in play. It seems like it would take a lot of resources. Of course I would happily take it, but I think Florian has other things on the priority list.
I even have some user-defined fields which are only meant for 1 character, i.e. a field for live-recordings which is only filled by me with a "1" if it is such a recording or the wellknown compilation tag. To use this benefit of a 1 character field you would have to define a label also with only 1 chracater like "L" for Live or "C" for Compilation.
Thanks for all your ideas!
I've just released Mp3tag v3.19a and added the Tag Panel field sizes Full, Larger, Large, Medium, Small, Smaller, and Tiny. I've also added ellipsis to field names that might get truncated on resize.
Great! This was #1 on my wishlist.
Mp3tag got significantly higher level of useability and user-friendliness.
Many thanks
Thank you Florian for the amazing updates! 
My donation has already been sent. This time a little bit more because some of these updates will really help. I only wish I could donate more. I am so excited about some of these features! 
As much as I LOVE this addition, it does seem like there is space for a smaller field than tiny. Something for 6 characters or less, which would be half the size of the smaller field. The Tiny field is about 66-75% of the smaller field. Maybe a minuscule, mini or tiniest field.
This reply in another thread from @Florian should explain the size decisions. Perhaps your screen resolution or panel size is driving the slightly wider width of the Tiny field size.
You are right. I am using a 38 inch monitor at 4K. I think I have a magnification of 110% so my characters are really tiny. For example, I can fit around 10 letters in the tiny field. I will read that post and consider a way of changing my workflow so these fields are more usable. Don’t get me wrong, this is a vast improvement to what we had before, but perhaps resolutions such as mine or not considered, when making these fields, which is understandable it is an outlier. I’m also going to try out these fields in other computers with different resolutions to see how things translate. Thanks for the insight. It is very much appreciated.