Hi!
Of course you can. But I just realized you have the search placed nicely into a wrapper div element. The search will always adopt to the width of this wrapper elements, so it might be more logical to set the width of this element with css. You can still use the custom css field for that. Based on your site I would suggest something like:
[html]
.header-search-wrapper {
width: 33%;
min-width: 200px;
max-width: 400px;
}
[/html]
So the actual width is given with a percentage value in addition with a minimum/maximum limit.