do you need to adjust the dimensions in the firmware or doe that get handled in the slicer?
Don't need to mess with the firmware.
Dimensions are adjusted under machine settings in Cura.
(Point of origin set by limit switches on machine, then dimensions in each direction from there are whatever is entered in Cura.)
While using the stock heated bed, I have a magnet stuck to the underside to trip the y-axis limit switch at the front edge of the shorter bed instead of where the longer glass bed would be.
The only PITA thing with this setup is the height change when stacking the glass plate on top.
Either the adjuster screws will have to be tightened down and re-leveled to lower the glass to the current height, or I'll have to rig up some type of spacer or adjuster on the z-axis limit switch.
I may end up just making a full length bed out of metal, but I suck at working with metal and don't want to spend money on an additional bed heater.
I am no expert on 3D printing stuff.