From skill 70 on, one order in five that you ask for is large. A large order asks for a whole set of one material, the same amount of each piece: 10, 15 or 20. You know it by its picture, a bundle of parchments.
Smithing has 63 sets: ringmail, chainmail, platemail, helmets, shields, maces and hammers, spears and forks, in iron, silver and the metals that are ordered. Tailoring has 62: the armour and footwear of each monster leather, and a few sets of cloth and common leather.
It is filled like a small one, with Add: a piece, a bag, your backpack or the order book. It also takes a full small order of one of its pieces in the same amount, which fills that piece at once if it holds none yet. Handed in, it pays the gold of all its pieces and twice the points: a set of Copper Maces & Hammers, 20 of each of its five pieces, pays 13,680 gold and 800 points.
Also, the nine rarest metals (EvilSpirit, Mystic, Scorpion, EarthSpirit, Moon, Titanium, AngelSkin, DemonSkin and Elfic) are no longer ordered, in small or large orders: gathering that much of them was too hard. Their ingots are still among the rewards.
Each trade's sets, in the bulk orders guide.