My 2 cents: The stock intake before the airfilter is 2 inches.
The problem of opening it wider and having the aluminum tube is that you increase the chance to flood the engine in case you decide to "submerge" and also the air intake will be too low. The stock setup is amazingly good with regards to avoiding the water to get to the pistons. I crossed rivers with water above the hood (praying, of course) and nothing happens; as matter of fact I got spilled spark plugs that disturbed the engine function, but after a couple of miles it got back to normal again.
About the snorkel, if you rely on the stock diameter, as you increase the lenght of the intake you strangle the engine air intake. I tryed that setup myself some months ago. Provided I didn't want to drill a hole in my fender (and also because the NG pressure regulator is in the drivers' side right behind the air filter box), and because of the wipers assemble, I tryed the longest way thru the passagers side. I raised the hood about 2 inches. Outside the visual was nice, but inside the hood... Anyways, I decided to quit for 2 main reasons:
1 - the air intake was strangled
2 - there was to much hot air coming into the car.
Here are some pictures of my weird experience...