Welcome to YT.
From what I have gathered here, Yorkies reach their full size at different ages: some as early as 8 months, others as late as 18-24 months.
For a clean bottom, do a "sanitary trim." Trim the hair around the anus. Stools should be firm enough not to stick to hair. If your pup's stools are too soft or runny, think about modifying the diet.