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To estimate the exit pupil, divide the objective diameter by the magnification. Therefore 10 x 50 binos give a 5 mm exit pupil, and 7 x 50 binos give a 7 mm exit pupil.
You can see the exit pupil, though, just by holding the binos a couple of feet away from you and looking for the size of the circle of light in the eye lenses of the binoculars.
If you're young and blue eyed, chances are your pupils will dilate to 7 mm in the dark, in which case, all 7 mm diameter of light in a 7 mm exit pupil will enter your eye.
If your older and dark eyed, your pupils mightn't dilate more than 5 mm, in which case, all the light from a 7 mm diameter exit pupil binocular won't necessarily enter completely into your eye. (I've heard the phrase "wasted light".)
Other people can tell you whether or not this actually makes a jot of difference...
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