If you purchased something on Grailed and the item does not fit to your expectations, the seller may not be required to accept a return.
Returns may only be eligible if the seller's description inaccurately described the item in terms of authenticity, color, condition, fabric, and measurement. If no measurements are provided, it is the buyer's responsibility to confirm fit and sizing prior to purchasing.
If your purchase was accurately described or did not have any description of fit/measurements to compare against, you should try to work out a mutual agreement with the seller so that they accept a return for a full refund.
If that does not work out, your best option would be to relist the item on Grailed to make your money back.