It is probably because the browser is trying to zoom the area since the font size is less than the threshold, this generally happens in iphone.
Giving a metatag attribute “user-scalable=no” will restrict the user from zooming elsewhere. Since the problem is with select element only, try using the following in your css, this hack is originally used for jquery mobile.
HTML :
<meta name="viewport" content="width=device-width, minimum-scale=1.0, maximum-scale=1.0, user-scalable=no">
CSS:
select{
font-size: 50px;
}
src: unzoom after selecting in iphone