I use balsamiq mockups for creating wireframe screens. I want to add a big calendar to the mockup. so i searched some icons in the balsamiq library and found a "calendar" icon. When i maximized the size of that calendar icon, the clear view of calendar disappeared and it doesn't fit my requirements.
Rather than using custom icons to design for calendar you can use a grid icon from the tools to make a calendar..
for example , try to drag a "grid" and then draw some lines in it which may look like a calendar. Try this. It will fit your requirements.