I have recently researched this and found a lot of good info at ...
http://www.parallax-tech.com/faq.htm
I compiled some of the equations into a calculator here ...
http://www.buildlog.net/cnc_laser/laser_calcs.htm
All of the parameters can be optimized, but usually at the expense of other parameters. I depends on what your priorities are...spot size, depth of field, etc. Check out the first link. It will probably answer all your questions.