Complexity...

I've seen a lot of articles trying to compare ITIL v3 and COBIT, CMMI, ISO/IEC 20000, ISO/IEC 27001... and describing ITIL as sometimes more, and sometimes less complex than others.

How to measure complexity?

I'm not sure I want to try to answer this question, I'm not even sure the question makes sense.

What is complex for me, in my context, might be simple for you, in your context.
Many factors such as background, experience, organization... contribute to perceived complexity.

Anyway, when you look for a method, you're actually looking for an enabler. If your perceived complexity of a method is high, then it's not an enabler.