Board 8'X 8' is no problem.
It depends how deep and how fast cut you want to make.
if you want to increas power, you can easily parallel the IRF mosfets
IRFZ44 is needed 8 to 10 + volts to drive.
ULNXXXX have sink outputs, say -ve (ground) output so they cant drive mosfet directly.
NEED HELP TOO...... I have a Plan on building a CNC Engraving machine and i'd to use DS89C450 as Microcontoller and 3 UC3770BN as stepper motor driver..... but I don't know how to interface this two parts and where will i connect every pins that UC3770BN has.......