While there's not one-size-fits-all, here is a basic recommendation for your designs.
Backgrounds
GoBrunch will always fit your backgrounds on full-width, with no stretching or cropping. If you don't want to generate scrolling, we recommend generating JPG images 1920 × 1080 or 1920x976 under 600kb.
For large auditoriums, scrolling is necessary and you can create backgrounds with a variable height to fit your needs.
For resolutions for portrait backgrounds (mobile), check this article
Canvas Height
Inside the Editor, you can see an icon the allows you to control your Canvas Height.
If you're looking to avoid scrolling, keep your height under 767. Some monitors have different aspect ration and that's why it might vary a little.
Images
While there is no specific resolution constraints, our recommendation is to keep your images under 300kb.
You can hold shift to keep the image proportion and aspect ratio.