To actually answer your question. The software doesn't know your xyz dimensions, you have to tell it. In your case if you want to build a DLP printer, max X and Y are determined by how far away from Z=0 the projector is mounted. Further away = larger print area but lower resolution while closer is exactly the opposite. Then set that maximum X/Y dimension in your slicer software. You will also need to set your Z max in your slicer software BUT you will ALSO have to setup your Z axis in the firmware in your printer for things like steps per distance traveled (inches or mm) and possibly max and min Z depending on what firmware you choose.

So to summarize, YOU tell the software what the XYZ max dimensions are, it won't know all by itself.

If you buy a kit or pre built machine the seller has done that work for you as far as the firmware is concerned. If they provide a slicer they have likely done the work there as well. If you buy a kit without a slicer then you will still have to set your max's in the slicer software.