How long should a graduate school statement of intent be?
It depends on the program, but if no requirement is set, I would go for about 500 words. That's about two pages double-spaced. You want to be to the point in these essays. Too much rambling may turn the reader off.

I never turn anything in single-space. Double-spaced documents help for readability. It's up to you, but I suggest double-spacing.