Inside your town units do not add any storage to resources. But if your looking to hoard resources out of the overfilling city, Laborers are the most cost effective resource holders. Each one hold around 200-300 resources I believe, this makes them very effective at trading resources with cities because they cost basically nothing.
Ogres can also carry alot of resources but laborers are the most cost effective, in the event your overstocked with resources, send some laborers out into an army, and trade your overstocked resources with them. You can then choose to either let them sit there or travel to one of the larger cities and trade for either gold or other resources.