Frequently Asked Questions About Light and Lasers Compiled by Dr. John Simonetti of the Department of Physics at Virginia Tech . Whether the laser is used for surgery or not, the mechanism is the same.
Larger lasers can produce more energy.
One can change the color by using different materials for generating the light. A helium-neon laser generates the familiar red laser light, other materials can generate green laser light, etc. I recently saw a TV news report where a laser was being used to remove skin that had been damaged by burns. The surgeon said the laser was more precise than a knife, and resulted in less scarring after the patient healed. That's not too far away.