max-width: 768px - change number to your required size
@media( max-width: 768px ) {
.site-header,
#site-navigation,
#sticky-navigation {
display: none !important;
opacity: 0;
}
#mobile-header {
display: block !important;
opacity: 1;
width: 100% !important;
}
.main-navigation:not(.slideout-navigation) .main-nav > ul {
display: none;
}
#mobile-header .menu-toggle,
#mobile-header .mobile-bar-items {
display: block;
}
}