Well, here goes....
1.) See QW-452.1(a) & (b) Notes 1, 2, and 3 ASME Section IX Boiler and Pressure Vessel Code. That would be a start. All downhill???

2.) see QW-452.3; 12.75 inch O.D. pipe will qualify a welder for 2 7/8 inch O.D and over per ASME IX. Also check out QW-461.9 for position and diameter limitations within the other limitations of QW-303. Your coupon size would be determined on what diameters and thicknesses you would need in order to be qualified to weld to the "WPS", preferably without any limitations. Now of course there can and will be some limitations on the part of the welder, for example, might not be qualified to weld in all positions and the WPS allows for "All Positions." That would be a topic for another day. I'm getting off course here. As far as test specimens go, Depending on the mechanical test that is being performed, 6” or 10” in length would be the minimum that some of the tests being performed for qualification would need. We would need to know what mechanical test.  QW-462.3 (a) or (b) is probably where you will find what you're looking for, as far as width and length is concerned for the test specimens.(QW-160’s Guided Bend Tests will point you in the right direction)

3.) YOU TELL ME??? My response is obviously referencing ASME IX.

That should help you out if you can locate the code book That I have been referencing. You have a lot more to go but you gotta start some where.

GOOD LUCK!!!