Sunnydale, California is the fictional setting for the U.S. television drama Buffy the Vampire Slayer. Series creator Joss Whedon

Joss Whedon

Joseph Hill "Joss" Whedon is an American screenwriter, executive producer, director, comic book writer, occasional composer and actor, founder of Mutant Enemy Productions and co-creator of Bellwether Pictures...

 conceived the town as a representation of a generic California

California

California is a state located on the West Coast of the United States. It is by far the most populous U.S. state, and the third-largest by land area...

 city, as well as a narrative parody of the all-too-serene towns typical in traditional horror movies.

Sunnydale is located on a "Hellmouth"; a portal "between this reality and the next", and convergence point of mystical energies.
Sunnydale's size and surroundings are implausible but justified given its origins — to sustain a human population for supernatural evils to prey upon.