You can use CSS directly in your SimpleXML. Add the following to your XML just above the </form> or </dashboard>
<row depends="$noshow$">
<html>
<style>
.input-dropdown {
min-width: 120px !important;
width: 120px !important;
max-width: 120px !important;
}
.splunk-dropdown .select2-container {
min-width: 120px !important;
width: 120px !important;
max-width: 120px !important;
}
</style>
</html>
</row>
