For your first quilt, I would focus more on what size you think you can finish. The first ones I've pieced are lap sized, with two that look like they'll finish as doubles much farther down the queue. The first one I machine quilted was a baby quilt - so I got to experience moving the quilt through the machine and maneuvering it.
The reason I would go with a smaller size is so that you can practice the various stages without getting frustrated or discouraged by something unwieldy. Eventually I plan on a queen for my bed, but for now, I'm sticking to the smaller ones.
Cheers, K