The exit pupil is the size of light cone exiting the eyepiece.

The light exiting  the eyepiece should fit inside your dark adapted pupil as it does with the 3mm yellow light rays above. An 8mm exit pupil (red ray) may be too large for your pupil and wastes light.

Exit Pupil = EP Focal Length (mm) / Scope Focal Ratio (f/#) or
Exit Pupil = Scope Aperture (mm) / Magnification

Examples:
55 Plossl (best suited to longer focal ratio scopes):
f/10 SCT = 55 mm / 10 = 5.5 mm
f/5 newt = 55 / 5 = 11 mm

or

35 Panoptic or 35 Plossl:
f/6 = 35 / 6 = 5.8 mm
f/4.5 = 35 / 4.5 = 7.8 mm

Examples at the small end:
5 mm eyepiece:
f/10 8” SCT => 10 /5 = 0.5 mm exit pupil, 406x power

Why 50x/inch?
1” = 25.4 mm => 25.4 / 50 x = 0.5 mm exit pupil

Department Store Tasco Power
60 mm scope at 675x = 60 / 675 = 0.09 mm exit pupil!

Many rules of thumb:
You often get the best contrast at exit pupils of 1 to 3 mm
Remember the 50x per inch of aperture inch limit for scopes
Nominal range for most people is 0.5 to 6 mm
Pupil of dark adapted eye typically opens to 6 to 7 mm
If the exit pupil is larger than your dark adapted eye, you'll lose light.