I have just recieved something I bought off eBay and i am quite dissatisfied with it

The listing gave detailed sizing, and I made sure to meausure before ordering and it is way, way too small even though I ordered on the smaller end of one size rather than the upper end of the other.

The quality is quite awful, but TBH I expected that with the price I paid, I'm just annoyed that it doesn't fit. So do I now leave negative feedback or do I email the seller? I have never had to leave negative feedback before - and I don't want any on me!

Any advice would be appreciated, thanks.