In Phoenix UI the lookup attributes in the properties views are always at max width, and we are not able to edit this, see an example (width of the lookup does not match the lookup values width):
Release : All Supported Clarity releases with Phoenix UI
If you would like the Lookup values to shrink to fit, this is not something that can be changed in the UI for Phoenix theme. The lookups' width is regulated (baked-in) within the theme code, it's not a setting.
You have the following options to get what you'd like:
1. Raise the suggestion to either have easier setting of column widths for Phoenix UI or lookups that shrink to fit on our Product Management Office Hours as enhancement
2. Create your own UI theme based off the Phoenix UI with your own changes to it. Unfortunately we at Support cannot suggest what exactly has to be done in the theme code as this is more of a consultancy thing. Changing themes to make them browser vendor/version compatible isn't trivial. You could also check on the Communities if someone has already done a change like this
Last option, if this bothers the users maybe it's best to use a different Theme such as Teal and Grey. Many users personally prefer it. Phoenix was created to be looking more like New UX, so if you're using New UX anyways it could be good to have both UIs look more different.