Tiny cut-out window panes are very attractive elements. But it is possible to design them too delicate to be cut out. The pane itself can be very small, but the crosspieces of the window have to be thick enough to survive cutting and the manipulation of construction. Experience leads me to recommend that the smallest pane size is 0.1 inch square, and the smallest crosspiece size is 0.04 inch.
It may not be desirable to cut out panes in a dormer, instead making them graphic elements. In that case, they can be designed smaller.