I am remodeling our bathroom and I'm almost to the part were I need to tile. I was wondering though since we want to use slate, if we could just use one big piece instead of having to use tile. It's a tub shower so the biggest piece is only going to be about 5' x 6'. Has this ever been done or is it a bad design?

As long as the entire stone is fully adhered to the substrate with a structural wall behind it, and as long as the edges are fully grouted to seal out moisture penetration, you can use as large or small a tile as you want.
I've seen marble slabs 100 sq. ft. used as a bathroom floor. And full height mirror walls.
If it's a thick piece of slate, just make sure the structure can handle the added weight.