You can cater in some media query so it only applies to your iPad mini.
@media only screen and (min-width: 600px) and (max-width: 900px) and (orientation: portrait) {
html, body {
font-size: 18px;
}
}
@media only screen and (min-width: 600px) and (max-width: 900px) and (orientation: landscape) {
html, body {
font-size: 17px;
}
}