I am afraid the font size of elements can only be changed using custom CSS.

If you send me a list or screenshot of the elements you want to modify I can provide you with their class names if you haven't already figured them out.

For starter give this css a try:

/* The whole top menu */
.stec-top * {
    font-size: 1rem !important;
}

/* Event preview title */
.stec-event-preview-content-title  {
    font-size: 1.2rem !important;
}

/* Event preview secondary text */
.stec-event-preview-content-secondary * {
      font-size: 1rem !important;
}

/* Event title */
.stec-event-title {
    font-size: 1.6rem !important;
}

/* Event description */
.stec-event-description * {
    font-size: 1rem !important;
}

/* Month layout day labels */
.stec-calendar-week-day-label {
    font-size: 1rem !important; 
}

/* Month number size */
.stec-calendar-cell-label {
    font-size: 1.2rem !important
}

/* Month event title size */
.stec-calendar-cell-event-title {
    font-size: 1rem !important
}