How thick of steel can a 6000 watt laser cut
Just know that the running cost of plasma is lower than laser. In general, it cost about 15/hr to run plasma cutting machines. It takes about 20/hr to run a laser machine.
To increase engraving area from 7.87"x7.87", to 11.81"x 11.87", you'll need a 50W or 30W fiber laser.