Hi Brent,
There should be no problem in using longer tubing to accomplish what you want. My suggestion is to remain with the same inside diameter as the original.
There is a simple method to test the accurate of an analog airspeed indicator and separately an analog altimeter using the "Water Tube Manometer." In fact, this method was exclusively used by the US Army and US Navy to check the accuracy of airspeed indicators and altimeters from the 1920s up and through WW2. Way back when, I have used this method on my Piper Pacer to test the accuracy of the airspeed indicator and altimeter while removed from the panel and again while installed in the aircraft as a comparison.
Jim Weir often writes wonderful "how to" articles for Kitplane Magazine. The following link is such an article on airspeed calibration using the "Water Tube Manometer."
http://www.rst-engr.com/rst/articles/KP89JUL.pdf
Russ Erb of EAA Chapter 1000 has also written a wonderful paper using the same method. By the way, Russ is an instructor at the USAF Test Pilot School at Edwards AFB, an experienced Aerospace Engineer, and builder of a wonderful example of a Bear Hawk. By the way on a previous post on this forum, someone provided a video link on vacuum forming compound curved acrylic/Lexan covers. The gentleman in the video explaining the method in his own kitchen is Russ Erb. That video should be titled: "Cooking with Erb."
http://www.eaa1000.av.org/technicl/instcal/instcal.htm
I have often wondered how accurate the electronic airspeed indicators such as the SKYDAT are and it would be simple to use the "Water Tube Manometer."
Now the second part of the problem is the location of the pitot tube. We all know that the pitot tube must be in mounted in "clear air." What most people do not realize is that the location dictates the length of the pitot tube to remain in "clear air."
All aircraft (ones that obviously don't have their engine in front of the nose) create a "bow wake" just like a boat and ship does in water. Inside the boundary layer of this "bow wake," the air is at higher pressure than the ambient air pressure outside the wake. A short pitot tube that is located inside this "higher pressure zone" would read at a higher airspeed. This is why Stan was reading higher airspeed indications than his gyro was flying. The stock mount and length of the pitot tube on the Sparrowhawk is too short to be in clear air.
One solution for the Sparrowhawk is lengthening the pitot tube to be in clear air, however a 3 foot long pitot tube is dangerous on the ground whereas anyone can inadvertently walk into it and cause damage to the aircraft and themselves. The other solution would be to mount the pitot tube on top of the cabin. Take a look at pictures of UH-1 Hueys and UH-60 Blackhawks and see that the pitot tubes are mounted on top of the cabin. The method that Stan used to solve his high airspeed readings was ingenious and very adequate since he didn't want the added expense or major alteration in removing and relocating his pitot tube.
Tom mentioned the pitot/static tube combination and it is the best type for lower speed aircraft. It consists of a pitot tube with a second lower static tube with and "capped nose" and multiple small diameter holes on the side of the tubing. The following picture is such a pitot/static tube system that is mounted on a 1930s vintage Bowlus Baby Albatros that a friend of mine restored and flies.
Anyway Brent, that is my long version to your simple question.
Wayne