Here is a photo of the back
You could cut a hole there about 10"w x 11"h
From the side
That space is about 10"w x 11"h x 52" long.
Except for the last half.
The rear wheel well takes up more than half of that space leaving you about 5"w x 11" h x 26".
So really you have one space that is 10x11x25 and one that is 5x11x25 although it is one large space with no partition. These measurements are approximate.
Hope this helps,
Eric