If you want to host your referral campaign on Viral Loops Pages, the recommended dimensions are the following:
“Milestone reward” image (recommended resolution: 520x520px)
“Features” images/icons (recommended resolution: 140x140px)
“Image with text” image (recommended resolution: 890x614px)
“Participation” background image (recommended resolution: 1920x1080px)
Keep in mind that these are just recommended dimensions and you are free to use whichever resolution you seem fit.