Are "pigtailed" conductors at an outlet required to comply with the length requirements of 300.14 for conductors that are spliced in the box?
No the requirement is 6” from the back of the box , no tape measure a dollar bill will , if you only have a 20 I will trade you for a dollar and you will be good,,, ok that has only almost worked once LOL. 
My first job as a licensed electrician my dad said I will save you thousands and went back and clipped all my wires on the rough in , the inspector knew I was just had just got my card and measured them, yes he had me replace everything that was less than 6 from the back, most of them I had some slack , but I did have to replace a few lines , ever since then I have always said it is better to waste a foot than be short an inch. , my latest graduate on his first job trued to save some $ on a parallel 500 job , oops now we had to add a j box and splices, I was upset with 12 awg ,, I think he will never do that again as it added quite a few hours splices and a very large j box. 
You cannot make them “whatever length”  6” from the back of the box or 3” extension I front I think is the minimum code.