This is a tricky one to solve - the popup is using the bootstrap popover script in Joomla and is created by javascript AFTER any CSS has been set to control its size and positioning
You could try the following tweak - edit the file components/com_jevents/views/flat/month/calendar_cell.php and change line 137 from
JevHtmlBootstrap:
opover('.hasjevtip' , array("trigger"=>"hover focus", "placement"=>"top", "container"=>"#jevents_body", "delay"=> array( "show"=> 150, "hide"=> 150 )));
to
JevHtmlBootstrap:
opover('.hasjevtip' , array("trigger"=>"hover focus", "placement"=>"top", "container"=>"#jevents_body", "delay"=> array( "show"=> 0, "hide"=> 0 )));