** Big vehicle = small penis: any proof? **
Absolutely! I drive a truck, and I’m a woman.
I have a Dodge Ram and a Scion tC.
I trade my penises out on the weekend.
We have a saying for that in Quebec: grosse corvette, p’tite quéquette.
As a former Vespa owner, I can assure you that it’s quite a babe magnet: “Oh, such a cute bike! Take me for a ride.”
I drive this. It’s small and zippy and awesome. Any other conclusions are left as an exercise for the reader.
I can’t find it now, but a year or so ago, I came upon a website called “Big Truck Small Penis,” or something like that. And I downloaded a bunch of bumper stickers that say, "Ask me about my small penis!"and I printed them on to some sticker paper. I confess that I have not had the opportunity to slap one of those babies on the back of a Hummer in the parking lot of the neighborhood grocery store, but I keep looking for chances. I would just LOVE to do that. Of course, part of that finding that chance entails being darn sure that the owner is a great distance away - not easy to do.
No, this is:
I still remember that from when fatherjohn (!) used to link to his “poseur 4x4” website. While that site is now defunct, they salvaged this.
As it says on the bottom of the page, this was a lot funnier before the International CXT came out. That is currently the standard bearer for size in a production vehicle, far exceeding even the Hummer H1 in every category.
With respect, that has no bearing on the question. Undoubtedly, certain guys who buy large trucks have large penises. Neither statement addresses the question put in the OP, which concerns a correlation between penis size and truck size. For that you’d need a nationwide survey.
A thought just occurred to me. Although I no longer drive and SUV, I still miss mine. Perhaps I do have a small penis. Not being gay or having been in a locker room since adolescence, I really have no basis for comparison. I had thought The Master had done a report on penis sizes, but I’m unable to locate it currently.
What constitutes small versus large versus average?
That’s brilliant. 
Men with a small penis would be unlikely to volunteer for such a study, which would skew the results. The only way to get an accurate answer would be to mandate participation by adding questions about car and penis size to the 2010 census. To prevent little lies, census takers would, of course, have to come around to measure every man’s unit.
I can just hear the Libertarians now.
I like the disclaimer: “Fits under MOST bridge underpasses.” 
I don’t know about penis size, but there is evidence that the auto industry thinks owners of large SUVs are insecure and in fact, tailor their marketing to feed that insecurity.
From the book High and Mighty: The world’s most dangerous vehicles and how they got that way:
There is an inverse correlation between male genital size and the amount of complaining the male does about the genital sizes of SUV drivers. 
I think your ability to apply these to as many vehicles as possible would tell me a great deal about your testicle size.
Yep. They are small. But I have three.
[Random Chuck Norris fact] Chuck Norris once challenged Lance Armstrong to a count the testicle contest. Chuck won by five.[/RCNF]
I heard a different version:
When Chuck Norris heard that his good friend Lance Armstrong was recovering from testicular cancer, Chuck gave Lance one of his. Lance went on to win the Tour de France 7 times. Chuck still had 2 testicles; no one knows if he had a spare or if a new one grew back.