This synthetic paper has made the best endpapers we've ever used. Tough, durable, resistant to buckling. Size is 450 x 640mm, 50 sheets to a pack.
We generally use this for making the endpapers for hardcover books of any size. The direction of the grain is not crucial with synthetic paper, so you can fold either way for good results.
The paper can be easily cut with a craft knife and a straight edge, such as our 500mm stainless steel ruler.