<!--Either inside your <head> tag (the #search means "the element with an ID of search")-->
<style type="text/css">
input#search
{
width: 20px;
height: 20px;
}
</style>
<!--Or inline, in your <input> tag-->
<input type="submit" id="search" value="Search" style="width: 20px; height: 20px;" />