Photos on this site come in three sizes:
1. Low resolution image up to 1Mb file size
2. High resolution image up to 4Mb file size, and
3. Original image
Low resolution images are useful for online, Facebook, web pages, etc. They cannot be scaled up to large sizes and are not suitable for printing.
High resolution up to 4Mb - these images are generally suitable for printing up to an A4 size.
Original image - most of the original images range between 14Mb to 20Mb in size, the dimensions are 6000 x 4000 pixels with a resolution of 350ppi. What this means is that you can print these images up to approximately A2 size - this does depend on the image though.
The original image is from wikipedia however I have superimposed some of the print sizes for the photos on this site.