"Vêtements" is masculine - that is why it is "petits"
"Ses" could be either "her" or "his"
"Ses vêtements sont petits" can be translated as either:-
"Her clothes are small"
"His clothes are small"
The gender of the adjective (petits) follows the noun (vêtements) not the person who owns them.
i guess u had replied "her clothing are small" had u replied "her clothes are small" ,your answer would have been marked correct. her clothing are small makes no proper sense in english , it is grammatically wrong . vetements can be taken as clothing or clothes according to the context of the sentence. hope this helps. :)
Generally, how do you know it is him or her? By knowing the person first.
So, the clothes belong to him/her. You say "Her/His clothes are small.
BECAUSE we are talking of clothes, they are many, hence they are NOT Son or Sa, BUT Ses.
Vêtements refers to a masculine noun. That is why we say "vêtements petits" and not "petites."
To wrap up (pun intended), we say: 'Ses vetements sont petits,' whether his or hers.