whats the height of the rear door opening?

 

The top of the rear opening is not flat square so a maximum height reading will not allow a square box of similar height to fit. You would need to tell us how wide the box is in order to take the best measurement. In addition, the floor depth isn't exactly the same as the frame. I will provide you some conservative measurements from both of my vans. A 2013/14 should be identical to a 2008 because body changes were largely cosmetic:


For 2008 gen 5 Grand Caravan sxt with stow-and-go:
side door openings: 27-1/2" wide, 43" tall
rear door opening: 36" wide and 37-1/2" tall. The max width near the bottom is a bit over 48", so it will swallow plywood. However the max height is much less for widths greater than 36" because the top of the opening is not square.


For 2001 gen 4 Grand Caravan (all):
side door openings: 27" wide, 43-3/4" tall
rear door opening: 36" wide and 37" tall. The floor on Gen 4 without stow-and-go is about 2" lower than the lower door frame, so the effective height of the opening is closer to 39".

One thing is for sure, the larger, prominent hood on gen 5 makes it smaller inside than the older gen 4 in every important way....less interior volume and less legroom. The side doors were increased in width on Gen 5 because the non-grand design was discontinued. This was a small positive.

 

thank you very much for the info.