try the !important
argument to make sure the CSS is not conflicting with any other styles you have specified. Also using a reset.css is good before you add your own styles.
select#wgmstr {
max-width: 50px;
min-width: 50px;
width: 50px !important;
}
or
<select name="wgtmsr" id="wgtmsr" style="width: 50px !important; min-width: 50px; max-width: 50px;">