It makes surprisingly little difference if the header images are too small, e.g. 940 x ca 200 px rather than 1200 × 280 which is what twenty sixteen recommends. At window widths less than the image width it makes no difference because the theme re-sizes the image anyway; at larger window widths the image left-aligns but doesn’t right-align. And image height makes no difference at all to the functioning of the theme, unless images of varying heights are used in a slider.
Some of my sample images are tagged with their sizes. Height varies, and (to me) 205 looks a bit miserly, 280 dominates the page too much. ‘Townsville from Mt Stuart’ is 235px high, while my paperbark blossom is 205px high (mainly because that was easiest, working from the versions I had). Both have 12pt Futura titles.