The angular size of the sky you can view through a telescope is called the real (true) field of view; it is measured in degrees of arc. The larger the field of view, the larger the area of the sky you can see.
Field of view is calculated by dividing the apparent field of view (in degrees) of the eyepiece by the magnification. For example, if you are using an eyepiece with a 50 degree apparent field, and the power of the telescope with this eyepiece is 100x, then the field of view would be 0.5 degrees (50/100 = 0.5).
Manufacturers normally specify the apparent field (in degrees) of their eyepiece designs. For a given focal length eyepiece with a given scope, the larger the apparent field of the eyepiece, the larger the real field of view and thus the more sky you can see. Likewise, lower powers used on a telescope allow much wider fields of view than do higher powers.
These two pictures show the same apparent field of view, but the one on the right has a much smaller real field of view:
(Photos courtesy Alan Hale)