I believe the file you are looking for is called, route_map.tpl, you can shange the size be defining and changing the default code at the top,
Just change from this,
<div id="routemap" style="width:<?php echo Config::Get('MAP_WIDTH');?>; height: <?php echo Config::Get('MAP_HEIGHT')?>"></div>
To this,
<div id="routemap" style="width:600px height:480px"></div>
That should give you want your after.