I have been using Proton Pass on Firefox for linux and I find that the extension popup is a bit on the small size. It looks better when I enlarge the entire scale of my desktop, but I would rather not do that. Being able to increase the zoom of the UI in the popup to someting like 110% would be very useful. Please consider this feature.

Existing ProtonPass extension (add-on) on the browser is not resizable, forcing to scroll down for more options to view.
There's a feature in Advanced > 'Open in a window' but it's not possible to set it as default.
So, either make it possible 'Open in a window' as default, or just make possible to resize the ProtonPass window.

Today I was able to open Proton Pass as a separate tab in Firefox. Hurrah! I can now increase the font and read all the text comfortably.

The browser extension allows you to open Proton Pass in a separate window; am I reading the question wrong?

I use firefox and I agree. Either allow using a whole tab or provide an additional standalone application aimed at editing/managing the passwords.
The small window is fine when searching for existing logins though.

The UI is way too spacious, and the window is really short

Trying to read a tiny little popup window with text that is dark blue or purple on a black is damn near impossible under most lighting conditions.
Just because CERN likes to build its biggest experiments underground, doesn't mean that Proton Pass users will be happy to crawl into a cave every time they need to use their password manager.

Same thing in Firefox on Linux

As things stand the window in Edge is too small to be able to see the full name of the Account, URL's etc. Very frustrating when you have similar names on multiple accounts.

Suggest make the window resizeable or allow it to be dragged to the side of the screen as a stand alone window