It is common to let the corners
of a grid or square "run off" the visible field as they may not be
needed for your application. And, in fact, you could order a grid pattern
larger than the physical size of the reticle, or optical field diameter of
your eyepiece, if you simply wanted to fill the field with the grid pattern.
For example, an S-14183 20x20mm grid of 10,000 squares on a 20mm physical
diameter (or smaller) reticle would cover the entire reticle.