The best shape for a button on a touchscreen is that of the finger. about 1.5 x 1.2 cm. Pretty square.
But if I make a combobox that size, I can less than 3 letters of each line of the 150 point combo text when it is opened! A combobox must be 4 or 5 times as wide as it is tall to see more than a couple of characters. And 10 to 15 times as wide to see a filename! That's a big waste of screen space. If you make the combo less tall, it becomes hard to select with a touchscreen.
Either the popup could be wider than the button, or the text could be adjustable to be smaller, either would be fine with me.....
cheers,
-e
