^ What Buggs said! Just an inch or two wider than your sidewall width (outside sidewall to outside sidewall) gives you a good compromise between utility, weight and useable power/traction. Since most of our machines don't have "downforce" on the 3pt hydraulics, your ability to dig in is limited by the weight of the implement per foot of blade width.
My personal preference:
> More weight is better. For a given width, more weight adds ballast, helping traction. It also helps the blade cut into the soil.
> Less width (to the point that your tires will sit in the bottom of the cut and not scrub the side of the cut) is better. Your FEL bucket may also set a practical minimum width, if it's wider than your rear tires.
> More traction will probably do more for your capability than more power - chains, loaded tires, etc.
-Jim