Hi Francesco,
I have two possible suggestions:
One is to simply change the search bar width to a static pixel value, instead of the default ‘100%’ width, for example, to change it to 500px width: https://i.imgur.com/iqHOodR.png
You can change it to any value of course.
Another possibility is to set a minimum width via custom CSS, so the actual width stays 100%, but there will be a minimum width the bar has to respect, for example:
[html].asp_main_container {
min-width: 500px !important;
}[/html]
Apply this code to your theme custom CSS field (if it supports it) or use the custom CSS field on the search plugin back-end.
Use whichever solution fits your needs better 🙂