You need to use media tag
Say for example if screen size is less then 400px then you want to fix the with of a class as 350 then you can try following -
@media only screen and (max-width: 400px) {
.qa-login-group {
width: 350px;
}
}
Also you need to add the viewport something like
<meta name="viewport" content="width=device-width, initial-scale=1">