Thanks to chromes element inspector, I can see that the infowindow content you provide is always wrapped in <div style="overflow: auto">..</div>. Setting maxWidth does not seem to stop the horizontal scrollbar if you have scrolling height. I decided I needed to hack the parent element and turn off the horizontal scrolling, ie overflow-x: hidden.
Put an id identifier in the html you provide, and find that element (using jQuery here) after the infoWindow is loaded (need to use addListener). Jump from the element to the parent and set its overflow-x property to hidden, and then horizontal scrollbar is removed.
This is of course a hack - and may stop working if googles infoWindow HTML code changes - hopefully by then there will be a better solution also.
//assume you have a marker already called marker
google.maps.event.addListener(marker, 'click', function(){
var _info = new google.maps.InfoWindow({content: '<div id="infoWindow">content that is vertically large...</div>' });
google.maps.event.addListener(_info, 'domready', function(){
$(document).find('#infoWindow').parent().css('overflow-x', 'hidden' );
});
_info.open( map, marker );
} );