Have a look at the streamlining template.
Thinking big is good, and you may want to think bigger. The distance from where the curved roofline begins to where it meets the ground (roughly the length of the boattail), is 1.78*D, where D is twice the height of the vehicle. In your case, that's 6.34m! That's obviously not going to happen.
But if you line up the template with the widest part of the TOP view of your minivan, you find that D is the width of your vehicle, 1.90m, and 1.78*D is 3.39m. That's still a little long, but 70% of that length (i.e. 2.37m) gives you almost as good of a result. See
this thread for drag reduction estimates for shorter boat tails. Even a 50% chop, 1.69m, looks to be good for a very large reduction in Cd.
I like the way it looks, and you'd probably have the least thirsty seven-passenger vehicle on this site. Very cool, and I hope you do it.