First, after trying both systems, I am going with a boom. It gives much better sail control and reduces sheeting loads.

The big advantage of a boomless boat is it sets itself going upwind but it is never sets itself quite right. Also, booms are expensive figure $500 retail or $150 if you build it yourself.

For foot lenght, the practical limit is about a foot aft of the traveler.

For efficency the foot of the sail angles upward (~30 deg). With long booms you get into diminishing returns plus it starts looking a little odd

As you sheet ou the CF moves aft. The CF runs from ~1/3 - 3/4 the boom lenght from the mast. Don't let the CF get too close to the rudder. I have only sailed one boat whose boom extended significantly past the rudders and as the owner warned me "don't sheet out too much in a puff, it gets strange"

Long booms are just awkward.