All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
It seems that wearing this dress to know its measurements is not considered a use that prevents the item from being returned. Therefore, if you adhere to the rest of the seller's exchange policy, it is permissible for you to acknowledge compliance with those instructions contained in the exchange policy.
It is better to explain this to the seller because whoever is truthful and straightforward will be blessed in his sale, and whoever conceals and lies, the blessing of his sale will be eliminated. As in the hadith where the Messenger of Allah said: “Both parties in a business transaction have a right to annul it so long as they have not separated; and if they tell the truth and make everything clear to each other (i.e., the seller and the buyer speak the truth, the seller with regard to what is purchased, and the buyer with regard to the money) they will be blessed in their transaction, but if they conceal anything and lie, the blessing on their transaction will be eliminated.” [Al-Bukhari and Muslim]
Allah knows best.