I came here to post exactly the same thing that you did!
I just upgraded to v2.90 from v2.78 (.... yeah, if it ain't broke don' fix it!) and while I like and very much appreciate the new features, stability and bug fixes, the "field size" issue instantly annoyed me within seconds of the install completing. It's like coming home from work and finding that someone has moved your furniture around - lol.
In v2.78 the 'Genre' field in the Tag Panel was about 2.5 times the size of the 'Year' and 'Track' number panel. I have no idea why all the fields have now been made the same size. When is a track number (usually a maximum of 2 characters) ever going to require the same amount of space as a field which describes musical genres? The same goes for the Year field which will always only ever be 4 characters (there is a separate Date field for people who want a full date). So it seems a strange choice to limit the Genre field so that most of the text it cut off, just to give a bunch of space to two fields that don't need it.
Like other posters here, I would very much like to see a greater range of field sizes, or more control so that you can specify the size yourself.
Additionally, I find the naming confusing and inconsistent. "Normal" is not a size, and gives no indication of how big it will be or how its size compares to other fields. "Normal" implies to me "standard single line" instead of "Multi-line", but it says nothing about its size. I also find the fact that "smaller" is bigger than "small" makes no sense. Small > Smaller > Smallest. Big > Bigger > Biggest.
Finally, the change in font has made all my tag field titles larger, so now I have to widen the tag panel (and reduce precious track-list column width space) to be able to fit the same amount of information that I used to have in my previous Mp3tag version.
Luckily, all these issues are really trivial to fix, and they would really improve the user experience considerably, so hopefully Florian would be prepared to make a few adjustments.
I'd also like a checkbox option on the Tag Panel settings dialog to lock the panel selection so that you when you right-click there is only a menu item to go to the Tag Panel configuration dialog to unlock it (if you want to make changes). For those people who change their tag panels constantly, then obviously they would not choose to select the "lock tag panel fields" option, which means that could continue enable and disable individual fields by right-clicking the the panel, just as you can now.
Thanks to the developer for all his hard work fixing bugs. I read the Version History list and a lot has happened since v2.78.