First, I think the title is misleading, the issue here is not the + button from the top bar, but the "Copy a Survey" button in the "Copy" bar, from the page accessed after clicking the + button from the top bar.

Secondly, this seems like an easy fix. While inspecting the button element I noticed that the "Copy a survey" is not of the same class as "Create a Survey" or "Import a Survey" buttons, so an easy and satisfying solution would be to change its class to match the two other buttons. It would in fact make the button a bit bigger.
I'd be happy to submit a patch to solve this.

If you submit a pactj PLEASE : remind A11Y (and UX) https://bugs.limesurvey.org/view.php?id=18746 : add text to button !

This issue was not properly solved with my initial merge request. The button's width was still not adapted for smaller windows, and affected the Copy button, but also the Create and Import ones.
I have submitted a new merge request solving this by making the button's width adaptative.

https://github.com/LimeSurvey/LimeSurvey/pull/4289