The diameter isn't the critical dimension. The wall thickness is, and as per AWS D1.1, the minimum wall thickness of 1/8 inch or 3 mm is the determining factor whether D1.1 covers the structural work or not.
The bottom of Table 4.10 (D1.1-2004) has a listing for "Tests on Pipe or Tubing". They list the size of the pipe as less than or equal to 100 mm and greater than 100 mm (diameters). Next they list the nominal test thickness and it is either unlimited (any wall thickness), less than or equal to 10 mm, or greater than 10 mm. If the wall thickness is less than or equal to 10 mm, the thickness range qualified is 3 mm to 20 mm. If the wall thickness is greater than 10 mm, the thickness range qualified is 5 mm to unlimited thickness. Make sure you read the foot notes.
I don't have my 2008 handy, but I assume it is very similar if not the same as the 2004 edition regarding welder qualification.
Best regard - Al