Iron Chests adds a variety of metallic chests with steadily increasing capacity and capabilities. Here is an overview of all the new chests:

Name	Rows	Slots	Extra Abilities
Copper Chest	5	45	None
Iron Chest	6	54	None
Silver Chest	8	72	None
Gold Chest	9	81	None
Diamond Chest	12	108	None
Crystal Chest	12	108	Transparent
Obsidian Chest	12	108	Indestructible

When you want a larger chest, you have two options: make a new chest, or upgrade an existing one. Making a new chest works exactly like you'd expect: put the materials on the crafting table, and you get the new item. But, this leaves you with the trouble of transferring items from the existing chest to the new one.

In that case, the better options is to upgrade an existing chest in place. To start, you'll need a an existing chest placed in the world somewhere. It can even have items already in it. Next, you create the upgrade you need based upon the chest you have (e.g., an Iron Chest) and the chest you want to upgrade to (e.g., a Gold Chest). In this example, you'd need the Iron to Gold Chest Upgrade.

Once you've made the upgrade, you just place it in your hotbar and right-click on the chest you'd like to upgrade. The chest will then change to be the new type in place, and without dumping any of its contents. There are quite a few different upgrades, but not every possible combination is represented. Here's a diagram showing which upgrades are possible: