Hi tuxedosam! Okay, I'm a bit confused as to exactly what you're looking for, so I hope you won't mind if I ask a few more questions to help with the specifics.
Are we talking about stretching exercises here? Exercises that will help your flexibility - which helps to increase your range of motion?
Or are we talking about how to develop muscles and strength in your limbs without bulking those areas up? When I hear the phrase "extension" work, I am normally thinking about developing long, lean muscle (rather than bulk) for extensions (developes, arabasques, etc.).
Last question, perhaps you are thinking of the concept of "lengthening" your limbs visually through the use of energy and correct muscle usage? For example, pointing your toes will "lengthen" your line (but only if done correctly).