you can use a combination of font-size and line-height to force it to go larger, but obviously only in the situations where you need the font larger too
edit:
Example -> http://www.bse.co.nz EDIT: (this link is no longer relevant)
the select next to the big search box has the following css rules:
#navigation #search .locationDrop {
font-size:2em;
line-height:27px;
display:block;
float:left;
height:27px;
width:200px;
}