Even though I have a QIDI X-Max, I don't use their slicer, which I understand is based on Cura's program. For 3D printing, one uses .STL format files, which are unit-less, but defaults to millimeter. It's always safer to build your models in millimeters.

7" is 177.8 mm, which is "close" to the 18.08 number you've referenced but not close enough for me to think it's a decimal point error. Same thing with your 5 mm reference, as 2" is 50.8 mm, although that's somewhat closer to a decimal point shift.

Consider to attach your STL file for others (including myself) to examine, process and expound.