The working area of DexArm at different height is different. The working area in Rotrics Studio is the max working area whose pen position is 0. If your pen position lower or higher than 0, the working area will be smaller. You can try to put something below the drawing surface to stack it to Z0 and you can get the max working area.
Here is the working space of DexArm in 3D space:
Here is the working area whose pen position is -90 mm: