From MozillaZine Knowledge Base

This article was written for Thunderbird but also applies to Mozilla Suite / SeaMonkey (though some menu sequences may differ).

If you want to change the width of the edit field used for quick searches (the control in the upper right corner of the folder listing) insert the following into your "userContent.css" file

#searchInput {
 -moz-appearance: none !important;
 width: 35em !important;
}

replacing the 35 as needed.