I don't know of anything smaller than the Mainstay lifeboat water rations. At 5" x 4" x 1/2" they're a bit larger than you're looking for but the smallest I've ever seen. I can attest to the fact that they're very durable, plus they're unaffected by heat or freezing. I've drank 5 year old packets that were subjected to 120+ F temps and -30 F, and they tasted better than my tap water.