For me I have about:
274mb for roughly 43 min (tv show) in SD
817mb for roughly 43 min (tv show) in 720p
They are of the same show but different episodes though so it could vary a few mb in either direction. You can pretty much just double those numbers for a 1hr 26 min movie.
Visually it's noticeable but negligible to me on such a small screen. I would rather be able to hold more videos that I will watch only once and then delete than hold fewer videos at a higher definition.
But that's a personal choice.