@sdetweil
Thank you for the invaluable tip. I studied the HTML elements using your suggested CTRL+SHIFT+I approach, and managed to increase the font size of the header month. This was the snippet I added to custom.css that worked:
.MMM-CalendarExt3 .module-header {
font-size:225%;
font-weight:bold;
padding: 0 0 20px 0;
font-color: #FFFFFF;
}