how to change only one of the text boxes padding width size in form

I custom added a dropdown box on my registration form but now I can't change the width on the css file because it affects all the other boxes style in the form too. I tried changing it in the public_html/plugins/user/profile/profiles/profile.xml but it won't work either. and the modified I only know how to change it in the index.html. when I right click on the page and select "inspect tool" to test it. test box is now But needs to be