I just got my diy mill running . But it seams being to run in minature..  The g code is in inches and mach3 is set in inches . Project is 5" x 12" but it engraves it about 1" x 2" . I am running a BL- usbmach v2.1 BOB.  dont know if conflickting with mach3 or do i need a different BOB . does anyone have help for me ?

Try running the Axis Calibration wizard (Settings page) just to check that you have your Steps per Unit set correctly for each axis.

Tweakie.