First of all we accept JPEG or TIFF files for printing.
In order to sell multiple sizes of a single artwork, you should upload your original, highest quality file. There is no need to resize your files or upload multiple files. This will allow you to create a single product with multiple size variants.
When you set up your size variants we automatically resize the print to the right PPI (300) and the right dimensions for each size variant. So just upload the biggest file you have, and let us handle the rest. Keep in mind that your file will not be cropped, so all the variant sizes will be the same aspect ratio as the original file. Learn more about this here.
The only time we would advise you to manipulate your file prior to uploading it is if it is not big enough to print at the sizes you want to sell it at. In this case you could use a service for upscaling files to create a large enough file size for the largest size you want to print, and upload that. Upscaling services are available in Adobe products, or you could try an AI upscaling service like this one.
