Should create gig as much as you can (must be your service related). I will help you to get more opportunity of getting more knock and ultimately more sell.
I will help you to describe your services in a simple way to your client. Help you to reduce conversation time and get more closer to your buyer demands.
The ideal size is 550 x 370. But for a retina display many image fall in short while🤔 You should increase image size 2X.
I mean 1100 x 740, this size serve quality gig image.