This is the Fiverr recommended size: 1280×769 but you could increase the gig image size but you have to adjust the image ratio size.
For example 1280×769 = { (1280 *3 = 3,840) * ( 769*3 = 2,307) } = 3,840 * 2,307
I hope you understand perfectly and it will help you.
Thanks.