Hi,
First, make sure that the search width is not set to 500%, that is not recommended at all. I recommend using the default 100% instead.
Then try these more generic rules instead. It will work with any search widget there.
[html].AjaxSearchProWidget {
min-width: 100% !important;
}
@media (max-width: 480px) {
.AjaxSearchProWidget {
min-width: 50% !important;
}
}[/html]
Depending on your needs, you will have to adjust the min-width numbers.