you can plan on it being about 4'. some get a little smaller and some get a little bigger, but 4' is about average.

I've never owned a MBK, but I've had a bunch of desert kings and many consider them to be the same snake. I think my smallest desert king was about 3.5' as a young adult (3-4 years) and my largest was a wild caught desert/speckled intergrade/intermediate that was a touch over 5'.