If you'll be knitting in the round, then you need to use a needle length less than the circumference of the item. For flat knitting, it would have to be really wide, like an adult blanket or shawl with a larger number of sts on it to need the 40". Unless it's over 48" that's way too much needle, it just gets in the way. You don't need it to be longer than the width of the item. I find 29-36" to be a good length for almost everything - sweaters, blankets, afghans.
sue- knitting heretic