A few other tweaks will be needed for those who are interested (i.e. content width, mobmenu.css may need similar changes, etc.), but this will get you started.
In style.php,
.mob_menu_left_panel{
width: 90%;
height: 100%;
position: fixed;
top: 0px;
left: 0px;
z-index: 99999999;
transform: translate(-100%);
-ms-transform: translate(-100%);
-webkit-transform: translate(-100%);
transition: all .30s ease-in-out !important;
-webkit-transition: all .30s ease-in-out !important;
overflow:hidden;
}
All the “%” except height were changed.
Hope this helps!