I read somewhere, (THX I think), that the ideal is a minimum of 30 degree field of vision, preferably 45 degrees. That being said, if you can get a good idea of what your field of view is in the theater, try to make your screen at home encompase the same view.
Next time you're there, place your hands verticaly against your face and slide your hands together till the sides of the screen are just visible, and remember exactly where your hands are. Then go home and place your hands in the same place.
At that point, you can sit at a fixed position and adjust what screen size you want, or with a fixed screen adjust how far back you sit to get the screen edges to just appear.
Personally, I sit 1/3 to 1/2 way back from the front.