You cannot change the size, it is hard-baked in the native Windows component that implements the calendar. Also obvious from the MonthCalendar class (same animal as the DTP dropdown), note how dragging the sizing handles does nothing at all. And how changing the Font has no effect.
Inflexible but not really a problem, any other program that runs on the user's machine displays the exact same calendar in the exact same size. If the user has a visual impairment that gives her trouble using the calendar then she'll change the system font size or video DPI setting.
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…