I have a set of NOS seats here and mine measure 63" wide by 12" high (at their highest point) and about 22" deep. My seats have the standard material on them...some of the Frazers used fancier material and stuffing so yours may be slightly larger. This should, however, give you some idea as to size.
I read somewhere that it takes around 9-10 yards of material to do the seats and doors. Hopefully someone else can verify that number.