This test below is a box 5mx2mx1m (sorry, I'm metric). Base wood texture image set at 1mx1m
it looks as though its all dependent on the scale of the base image used in the texgrid fill colour. If the base image is say 1mx1m (as defined in the sketchup material editor)then that is the size of one panel.
test07.jpg
in this one I have adjusted the TexGrid repeat to 0.5, so each grid is 500mmx500mm
test08.jpg
and in this one I set the base image size in the sketchup material editor to 2mx2m (UV repeat back to 1.0)
test09.jpg