I am using endstops but how to I tell the marlin firmware how large my area is so thta when I home it knows to go to the end and stop? its going about half way now.
Thanks
Typically we do not use endstops.
I don’t use end stops with my MPCNC, I just move the tool to a good place for 0, 0, 0 and reset those locations with the buttons in bCNC. I don’t think your firmware would want to know the size of the work area directly, but maybe some CAM software has settings like that? I didn’t find anything in EstlCam like that though.
Knowing that I’m zeroed over a corner of the material and that what I’m going to cut will fit within the material seems to be enough at this point.
I don’t know if this is helpful. 
One last thing: the firmware might need to know if you’re homing to min or if you’re homing to max.
Usually if it’s stopping half way, or anywhere that’s not engaging the stops, you’re getting electrical interference. Mine does that occasionally. I just hit home again. So far I’ve never had an issue during a cut, not sure if the firmware even recognizes end stop hit during a cut.
There are defines X_BED_SIZE and Y_BED_SIZE in the configuration file that are used to set the coordinate limits (X_MAX_POS etc.). I think they get ignored unless you define software end stops.
You are correct, I was hopeful that I could have the software somehow help me to calculate that.
I dont have any electrical issues, that is not the cause.
I believe the values are in millimeters. There is a comment in the code that you can also set/probe the values with the M211 command.