You need to ask for measurements. It's always important to ask this if they are not posted because you as the buyer, need to take initiative and ask questions if you need to know of a garment is going to fit you or not.

Poshmark states that you cannot return an item if it simply doesn't fit. You can only turn the item if it's not as described.
If you received an item and it's described exactly how the seller described it, you're stuck with the item if it doesn't fit. The only thing you can do is Re-Posh it and hopefully get your money back.