From your second post in this thread, you have answered your main question.
Having a high proportion of returns will affect your rating. However, you should have other concerns regarding your account.
You should accept the return of the wrong size jacket.
Currently, your returns policy is illegal and goes against the Consumer Rights Act. If buyers make an online purchase from a business, they have a legal right to return it for any reason. You are clearly trading - nobody has 1500 unwanted items of clothing in their home (current items for sale plus your sales this year are about 1500) - and you state in your item description that your clothing is 'hunted down'.
You should upgrade to a business account. Private accounts are for selling your own unwanted items, not hundreds of items bought to sell.
See the link in the reply from @four_mid_able to change your account.
You should urgently add minimum 14 day returns to your items.
You should make sure your accounts are in order for when HMRC come knocking.
"There are two ways to be fooled. One is to believe what isn't true; the other is to refuse to believe what is true.”
Søren Kierkegaard, Danish philosopher (1813 - 1855)
