When I created the Plain theme (I modified one of the other themes) I had the source file monthstyles.css from the theme folder open, then the Calendar programme with a calendar that I exported each time I had made some change. Then I also had the browser with the exported calendar.html open. Obviously I had to refresh it for every export. That way I was able to see what impact my changes had.
As to the font size of the events, I haven't found it anywhere in the source file, but in the exported monthstyles.css it's in this chunk of code:
/** styles used in all events **/
#cc_eventlist .Calendar_1, .Calendar_1allday, .Calendar_1left, .Calendar_1right, .Calendar_1leftright, .Work, .Workallday, .Workleft, .Workright, .Workleftright
{
position:absolute;
top:0; left:0;
display:block;
overflow:hidden;
z-index:99;
font-size:11px;
font-family:arial;
white-space:nowrap;
cursor:pointer;
text-shadow: #C0C0C0 1px 1px 1px;
}