Hi!
The problem there is that the container element, in which the search is placed has a very small width, and since the search width is set to 100% by default, it can’t be wider as the container element.
The fix is very simple, just set a static pixel width on the back-end, under the Theme Options -> Overall box layout panel, like so: http://i.imgur.com/EQuemWe.png
That should hopefully help!