I am going to be using a workbee 1500 x 1000. This size is not available when setting up machine. What do I do?
I believe if you click the machine tab you can change work area there to 1500 x 1000.
That is where the problem is. You need to enter rail size and 1000 x 1000 is the largest size available. Maybe because that is the largest X-carve?
Machine size is basically only relevant to soft limits and GRBL parameters $130-132 control those.
Soft limits are only relevant if you have homing switches installed, enabled and utilized.
Just pick the 1000x1000mm option and get the machine up and running 
Then edit whatever values you need