Will a 4x8 sheet of plywood or sheetrock into the back of the x5?

Not sure about those exactly but I've been able to fit some pretty big things. 42" Plasma With box. That was probably the highest thing. As for the widest, some logs cut up to use for landscapping. Needless to say, the 2nd row seats were folded down on both occasions.

the seats fold down so i think it would. If you had to you could leave the trunk open.

I have tried and the largest with the hatch closed is 48" x 60". The board was not able to lie flat either.