Width 39" in the front area before it gets wider towards the back. The back section is 55" at the widest, but that area is very variable.
Depth 42" in the center, but this is only at the floor level. The seats slant backwards, and the rear hatch will start to take up room once it is closed.
Height 16" to the bottom of the (removable) shelf.
The area towards the front of the trunk that is consistent is a 39x21.5" rectangle (by 16" high to the bottom of the shelf) If this is cutting it close, you should also know that the floor of the trunk curve upwards at the seat hinge, so some of your height will be gone in that area, and don't forget that the seats also tilt backwards so you don't get a full 39x21.5x16.