Thanks to Kevin B for pointing me in the right direction.
Changed width:'auto' to width:'100%'
var windowWidth = $(window).width();
$('#helpDialog_ABOUT').dialog({
width: '100%',
position: { my: "right bottom", at: "left bottom", of: $(window)},
draggable: false,
autoOpen: false,
modal: true,
show: {
effect: "drop",
direction: "down",
duration: 1000
},
hide: {
effect: "drop",
direction: "down",
duration: 700
}
});
$('#helpButton_ABOUT').click(function(){
$('#helpDialog_ABOUT').dialog('open').width(windowWidth);
});