Hi there,
You can try playing around with this CSS for the width.
nav#generate-slideout-menu {
overflow: hidden;
}
/* Desktop Off Canvas width */
body .offside--right.is-open, body .offside-js--is-right .offside-sliding-element {
-webkit-transform: translate3d(-500px,0,0);
-moz-transform: translate3d(-500px,0,0);
-ms-transform: translate3d(-500px,0,0);
-o-transform: translate3d(-500px,0,0);
transform: translate3d(-500px,0,0);
}
body .main-navigation.offside {
width: 500px;
}
body .offside--right {
right: -500px;
}
.offside-js--is-right.slide-opened .slideout-overlay button.slideout-exit {
right: 500px !important;
}