The size of your banner depends on the type of template you are running in GiveCloud. If you are using the free template included with your shop, we recommend your banners be 1170w x 430h.
Some more tips:
Save at 70 percent image quality.
Looks the same but loads up to 20 times faster.
Don't forget to link your banner images.
They can link to anything in you GiveCloud site - e.g.; products or product categories OR to other content such as your main website or facebook page!