when preparing to backup ipad what size should I check for photos? Original size or not?
What size should you check when using icloud to back up photos
Original Size will preserve the original quality of the photos but will take up more storage space. Other options most likely compress the images. While compression won't cause a dramatic reduction in the quality of the images, they will be less detailed than the original images.
See Set up and use iCloud Photos – Apple Support for more info.
Hope this helps.