The layout grid allows you to change the size of the column depending on the device. (phone tablet or desktop)
You can also customize your css depending on the size of the screen.
for example for an element with the class name of reponsive-testClass you will put something like that (in the file custom.css or main.css) :
.reponsive-testClass {
width:1200px;
}
@media only screen and (max-width: 600px) {
.reponsive-testClass {
width:400px;
}
}
@media only screen and (max-width: 300px) {
.reponsive-testClass {
width:200px;
}
}
In mendix 9 (and I think also in 8) you can select the width of the columns in a layout grid based on the device.