Nobody makes 40" wire, let alone a cage with 40" dimensions. The largest pans I've ever seen offered for stacking cages are 30" x 36", which works fine for a large breed doe with a litter.
As animalmom said, it isn't just the rabbit you have to think about - the cage has to work for you, too. A cage that has parts of it that are so far from the door that you practically have to climb in to get the rabbit isn't a good idea, nor will cleaning it be easy. 30" front to back is about as far as is reasonable for most adults, if kids are going to be doing most of the work, going shorter and wider might be a better option.
Bossroo brings up a good point - and it isn't just buck urine that you have to worry about. Bucks spray and splatter everywhere, but does often hike their butts as high as they can in a corner to pee, which can anoint things as much as a foot away from the cage. If you build a metal shed to house your rabbits in, ventilation becomes an issue. Rabbits drink and pee a lot (particularly lactating does) so moisture and ammonia are much greater issues than they are with chickens. In a confined space, that can mean that you need to clean at least a couple of times per week. Of course, the pans make it easy, as long as you don't go too long between cleanings and let them get too heavy.