Each image is not the same qualitatively. Each image has a pixel dimension of width and height. To determine whether a file is suitable for printing, divide the pixel dimension of the file by 300.
File dimension can be found in Apple OS by right clicking on the image file in the Finder. Click on Get Info and you'll see the pixel size under Dimension, e.g. 6400x7800. In Win10, right click on the file and click on Properties. Click on Details where you'll find Dimensions.
In the case above, divide 6400 by 300 = 21.3 and 7800 by 300 = 26. Thus, the image will print well at any size up to 21 x 28.
