Just had a look at a pic I got from the Hull game
2 lines on the pitch = 6 yd box (ie, the length of the lighting thing we see on the pitch where the goal would be, in that pic).
Looking at the pic, it would seem the height of the screen is about that of the two lines on the pitch, or the lighting thing. 6 yds = 5.48m. The screen is to be 60m sq. so 10x6m.
Yep, that looks about right, to me