Should I bye a 42 or 50 inch for a room about 6 by 4 metres? And is surround sound necessary?

I've just renovated my room (Knocked out a wall making it 6 x 4 one side and 4 x 3 another). I'm wondering about getting a flat screen for the 6 x 4 side but I can't decide between 42 or 50 inch plasmas. Theres only a $50 AUS difference but I don't want to cramp my room or use to much electricity. And also is surround sound necessary for a room of that size? Thanks guys :)