Hi,
I see you have it set to 60% width currently, so it will follow the container element. I believe the best solution would be to add a min-width attribute via CSS, so it will never get below a certain width. For example:
[html].asp_main_container {
min-width: 320px !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.
I hope this helps!