[gembud] Subset grid is too large error from gdcntr

I was attempting to contour sknt(mag(wnd)) from a global CED 1/8th degree 
resolution grid over the whole globe, and got this error:

GEMPAK-GDCNTR> [DG -41]  Subset grid is too large.

What can I do to fix this?  I've already set LLMXTG to 93,312,000, and LLMXGD 
to 21,000,000.