If there were an even smaller 1/6th width field I guess it just depends how much truncation one would want to tolerate. On the surface it seems like it'd just be half the usable width of a 1/3rd width field but with the added margins and such there's less usable space than one might expect.
For example based on the Tag Panel width in the OP and accounting for the field margin, padding and dropdown arrow the value in the OP of just the Track would actually be truncated (the stem of the
1 wouldn't be visible).
With horizontal Tag Panel layouts this would be less of an issue since it's more practical for wider column widths and generally more fields visible at once (which is why I like it), though from user screenshots I've seen over the years the majority appear to use the default, narrowish vertical Tag Panels similar to OP screenshot, which may limit its potential for general use.
Just some expanded thoughts on it. Not that I'm against the idea but also not without some notable drawbacks.
One other thing, about labels on small width fields, is there aren't any tooltips when hovering over them with the mouse to show the full label name (like Windows does for truncated filenames in a list view for example, or truncated browser tabs, etc), which is another drawback to consider since any custom label longer than a handful of characters wouldn't be readable. I think of my own custom tags for instance which only use a single character value which might benefit from shorter fields but which have longer labels. 1/4 width fields could strike a decent balance though.