Image size of multiple photos
It all depends on your goals, the composition of glued photos can be of completely different shapes and sizes. All Ellty template sizes are already the correct size so you don't have to worry about sizing.
Photo Collage image format
The ideal image format of multiple photos is JPG or PNG, download it in the highest resolution for the best display quality. If you create for printing - download it in PDF format to send to print.
Why you need to combine photos
There are situations when you need to combine several photos into one, for example, you want to capture the memorable moments of life in one photo, easily create memorable images using the Ellty online service.