You need to use a mouseout as well.
$('#divMenu > #menu li a').mousedown(function(){
$(this).css({
'font-size': '25px',
'color': 'red'
});
}).mouseup(function(){
$(this).css({
'font-size': '30px',
'color': 'black'
});
}).mouseout(function(){
$(this).css({
'font-size': '30px',
'color': 'black'
});
});
Your event code sets the css when the mouse is clicked. Once you move away it can't register a mouseup. But on leaving it will register a mouseout event.