If you are totally unfamiliar with sewing - keep it fairly small -
I find what I call couch size to be very useful - and I could still get it through my machine - approximately 60 x 80 (or up to 65 x 85) - like Amma said. This is absolutely the largest size that I would suggest for a beginner.
One reason is to keep the cost down - before you spend a lot and decide this isn't as much fun as you thought it would be.