It's too soon for me to tell if will "spoil" after opening. I'm not sure why it would but I don't know. They're inexpensive enough that you could replace them frequently, using up aging stock for the intended purpose of lighting your grill. Sealing them with something like a Foodsaver would probably also work. As for size the cubes aroue about 1" or so. They are somewhat crumbly not in a dry, chalky way. They're easy to break up or cut but they hold together pretty well.