Good article on this here: https://thingsthemselves.com/no-input-zoom-in-safari-on-iphone-the-pixel-perfect-way/

/*Solve issue of safari on iphone zooming in on text boxes when selected*/
/*It does this if text size < 16px, so to cure we'll just set all to 16px*/
.my_form_my_info input[type=text], .my_form_my_info input[type=number], .my_form_my_info select, .my_form_my_info textarea {
  font-size: 16px;
}
.my_form_setting input[type=text], .my_form_setting input[type=number], .my_form_setting select, .my_form_setting textarea {
  font-size: 16px;
}