Any way to reduce the size of the card cover on Trello?
It seems like the only option is to remove it.
You need to attach a picture with a greater width/height ratio. Try using a resize service like http://cloudinary.com/ when attaching.
Trello will always try to show your picture with the width of a card, which is fixed. If the image has a low width/height ratio, the image will stretch to the top until it makes the width equal to the card width, looking bigger. And vice-versa.