[RobbieZ]

Only the word 'notifications'?

Search in vbulletin-chrome.css for this

.notifications {
	font-size:{vb:stylevar small_fontSize};
	color: {vb:stylevar toplinks_link_color};
	-moz-border-radius-bottom{vb:stylevar left}: 5px;
	-moz-border-radius-bottom{vb:stylevar right}: 5px;
}

Change the font-size

Your are basically unbracketing the font-size stylevar call to your own unique size, changing the stylevar would change anything else it would be linking it to. Changing the source is a much better option I feel.