Actually...entering non-number values for grid size does the exact opposite: you have NO space to place anything!
You can, however, try to enter very large numbers for grid size, but as soon as you click "Course Info", they revert back to 999 x 999. I'm not sure if it works if you do not click Course Info, though...
EDIT: OMFG, testing a 1500 x 1500 level is trippy as heck! o_O