The load length (length of floor from behind the back seat to the rear doors) is 2059mm of the model to 2006. But as you get higher off the floor that length decreases as the backs of the seats 'encroaches' into the load space. As has been mentioned, you will have a little space above the driver and passenger cabin area you may be able to utilise.
The model van that Tosh has just bought only came in 1 length, although a LWB version was available on the continent.
It is the latest van version after 2007 that has 2 body lengths.