For this example, I am using Image A:
-4000 x 6000 pixel dimension at 300 pixels per inch.
-The aspect ratio is 2:3 (1.5)
-Available print sizes for this image which will provide a full frame crop are as follows: 4x6, 6x9, 8x12, 10x15, 12x18, 16x24, 20x30, 24x36.
Oh yeah, make sure to have the Pixel Chart on hand as well. As we look at the required pixel dimensions on the Pixel Chart we can see Image A will be able to print up to 20x30 (would print at 200 dots per inch). I would most likely opt to print at 16x24 and obtain the higher print resolution of 250 dots per inch.
*In most cases, any modification made to the image size or resolution within an image manipulation program creates a blurry or distorted image anyways as pixels are created or invented. If the resolution of an image is modified this will falsely bypass the low resolution detector on the website and ROES. It is best to work with the images' original pixels and native resolution.