The autoStretch property (boolean) specifies whether the modal dialog height should be increased automatically to avoid a scrollbar. The height is increased until DayPilot.Modal.maxHeight is reached.
DayPilot.Modal.autoStretchtrueconst modal = new DayPilot.Modal({
autoStretch: true,
maxHeight: 600,
// ...
});
modal.showUrl("/dialog.html");