What is the best image size for printing?
All images should be 300 dpi. DPI is the amount of ink dots per inch; 300 dots per inch is the required standard for printed material. Images should also be placed at 100% size in your final document. For example, if your image is 50mm x 50mm at 300dpi, then it is also that size when placed in your document. Lower resolution compromises image quality and may result in pixilation (where the pixels, tiny squares or dots that make up the image, are apparent when printed).
Please note that opening a 72 dpi image in Photoshop and simply changing the dpi to 300 will not increase the quality of the image.