The only job I’ve done that used the whole machine dimensions was/is surfacing my spoil boards. I did manually map it out for that.
You can use the machine to measure it’s own cutting area like you were thinking. Put the machine in the near left corner. Home there if you have switches or jog there till it grinds and back of 0.5mm. Then set X and Y to zero there. Now jog the the far right and stop before you hit the end, the X number in gSender is now the machine width. If you hit the end and make it grind the numbers will be off. Then do the same jogging to the back and you will have the machine dimensions.
The machine dimensions determined above would be the maximum area reached by the center of the bit. The actual shape that you could cut out would be less by the bit diameter, half on each end/side.
As far as marking your spoil boards, as you can see in this post, I’ve marked up mine with a grid that helps me out. I used a laser but you can use an engraving bit, if it helps you out go for it.
EDIT: In case you haven’t seen these videos that Sienci put out on making a guitar. Building a Guitar on a CNC.