You can check on rodent pro for there sizes,but we wean ours at 4 weeks.After that it just at matter of how much and what you are feeding them with the rate that they grow and your definition of a small rat you can get average size for small rats of rodent pro.Would say with ours it takes 3 to 4 weeks to reach small size depending on the size of the litter larger litters take longer to grow as they have to share the milk supply from mom till they are weaned.