Add this code to the Custom CSS field on the Settings tab of your Timeline widget’s settings:
.es-event-card-content-label {
font-size: 11px;
}
.es-event-card-content-title {
font-size: 21px;
}
.es-event-card-content-text {
font-size: 14px;
}
.es-marker-label-title {
font-size: 20px;
}
.es-marker-label-text {
font-size: 15px;
}