You can add custom CSS to the custom JEvents CSS file using the left menu item in the backend
Something like this would help
#extcal_minical .extcal_daylink, #extcal_minical .extcal_sundaylink, #extcal_minical .extcal_busylink, .extcal_weekdays, .extcal_navbar td {
font-size: 14px !important;
}