The annoying small popup-menu of the 1Password browser extension, where all item titles get cut off and an ... is added, can be modified to a normal sized box.

First, try it inside the dev-tools in the browser window:

Before:

After:

So that works. But where is menu.css? Right-click on the name and choose 'Copy link address':

That gave me: chrome-extension://aeblfdkhhhdcdjpifhhbdiojplfjncoa/inline/menu/menu.css.

I use Vivaldi, so I have to look here for that ID 'aeblfdkhhhdcdjpifhhbdiojplfjncoa' : /Users/myusername/Library/Application Support/Vivaldi/Default/Extensions/
You can find the exact folder for your Chrome-based browser, if not Vivaldi, under the URL chrome://version.

I found it and the complete path to 'menu.css' is now: /Users/myusername/Library/Application Support/Vivaldi/Default/Extensions/aeblfdkhhhdcdjpifhhbdiojplfjncoa/8.12.10.36_0/inline/menu/menu.css

Open the file in a text editor and find html{width:300px;max-width:300px} and change that into html{width:max-content;max-width:500px}.

Save and close, reload the page in the browser and voilá.

You'll probably have to this again after every update of the 1Password-extension, until they decide to make this change permanent.