Hi,
You can change the General Options -> Mobile behavior panel: http://i.imgur.com/Hp7Wff2.png
It’s hard to tell based on a screenshot, but my guess is that the problem there is that the search width is set to a percentage value by default (100%), and the container element might be too narrow.
I usually recommend two other possible solution to this case:
1. To set a pixel, fixed width for the search bar: http://i.imgur.com/Eh7OHaC.png
2. ..or keep the percentage width, but force a min-width via a custom CSS rule, like so:
[html].asp_main_container {
min-width 280px;
}[/html]