Your holes should already be "dimpled" for flat head screws. Choose a size that will allow the head to remain even with the surface of the window so that there is no sliding interferrence with the window action. You should try to get as long a screw as you can in that diameter. Use a bar of regular bathroom soap scraped against the threads of the screws to help them sink into the wood easier.
"True eloquence consists in saying all that is necessary, and only that which is."
François Duc de La Rochefoucauld