Hi, and welcome to JA!
There are about 25 million acres of desert in California. That's about a quarter of CA's total area!
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That's a much tougher question.
The Mojave, which is typically mountainous, is 22000 square miles (14+ million acres) http://en.wikipedia.org/wiki/Mojave_Desert
The Colorado desert (in California) is 2500 sq miles (1.6 million acres), and it is mostly flat.
If you'd like, I can opt out and have someone else take a look at this.