Fiverr gig image size should be 550 pixels by 370 pixels (minimum) as per Fiverr recommendations.
But if you use the minimum size, it would be blurry.
So as per my recommendation, please use 2750 pixels by 1850 pixels. It's 5 times larger than the minimum size. So remember to multiply the font size 5 times as well. It means, if you use 12pt font size, now use 60pt font size.
I hope it helps.
Thanks