at 45 degrees a 6' blade will cover just over 4' wide and a 7' blade just under 5'.
so it depends on your soils, and what you're planning on moving with it since I assume your CK25 is between 48-54" wide to the outside of the tires.
considering how much your tractor doesn't weigh, I'd probably advise going with the 6' blade to make it less likely that you'll try to pull too much at once.