Garry,
I had a 12/15 seater Ford E350 for 3 weeks last November.
Until that point i had never driven an automatic and the biggest car i had ever driven was my wifes freelander 2.

If i am honest i was a bit apprehensive about it, even though i consider myself to be an excellent driver (not sure how that sounds!) and when i first got into it i didnt know how i was gonig to get it out of the car park!
But i did and i took it nice and easy when leaving the airport and after about 15 minutes of the drive to Disney, i felt 100% comfortable in the car, and felt like i could drive it anywhere.
This made me relaxed and from then on, i was driving it with as much ease as my car at home.

The size of these cars is daunting, but in the end i absolutely loved it!