Can I use frozen spinach? If so, how much?

I would think it would be fine, however make sure that you thoroughly squeeze all of the water out of the frozen spinach, as most store bought frozen spinach has quite a bit of liquid in it, as will home frozen if it has been blanched before freezing. If you don't, all that extra water released could change how your dish comes out.

I haven't tried it, but you should be able to use the same weight (1 pound) of frozen spinach and treat it the same as the fresh.