Pleasantville is a very tiny world. Then how did TV channels obtain the image of the Pyramid of Khafre?
Pleasantville was a microcosm of a single town initially and limited to just the town limits.
Geography lessons at the local high school are limited to subjects like "Main Street" and "Elm Street" because the world literally ends at the city limits. Space twists back upon itself in Pleasantville, and "the end of Main Street is just the beginning again."
As the changes in the Pleasantville universe take place and people in Pleasantville begin to explore hidden abilities and revel in their new freedoms (as indicated by the appearance of color) so the universe expands.
Eventually, the entire town becomes colored, and the people of Pleasantville are finally introduced to the rest of the world. Televisions at the television repair shop now display full-colored images of various scenic vistas around the world, and Main Street, which had previously been a circuit that led back to its beginning again, now leads away to other towns and cities.