Small hands and bodies have limitations, and your child's attention span may not be geared to all of the tasks you'd like him to perform. Safety can be an issue as well. Before you start doling out chores, define what's appropriate and what isn't. Young children shouldn't be carrying or cleaning breakable objects, and even older children should be exempt from using harsh cleaning solvents or potentially dangerous tools. Once you determine that a task is safe, try to assess how long it will take and how potentially frustrating it will be for your child to complete.
Children do best with chores that are short in duration or that can be done in a number of easy, satisfying steps. The older the child, the more he can be expected to do, but you're the best judge of his ability to complete an assigned task successfully. Even though getting the chore finished is important, giving your child a sense of accomplishment and mastery over the task is important, too. One way to be sure you're doing your part is to inventory the supplies he'll need and make sure everything is available and ready to go.