Resize an image to 2480 x 3508

Whatever you choose. Crop to fill scales until the 2480 x 3508 box is covered and trims what hangs over, so you get the exact size with the edges gone. Fit inside scales the whole picture until it fits, so nothing is lost and one side comes out shorter than the box. Stretch forces both numbers and changes the shape of everything in the picture. The default is crop to fill, because an exact size is usually the reason somebody came.

It gives the picture the right number of pixels for A4 at 300 DPI, which is the part a printer needs. Whether it *looks* like a 300 DPI print depends on where those pixels came from: enlarging a small photograph to this size produces the right count and the same real detail, so it will print soft. Starting from a photograph that already has several thousand pixels on its long edge is what makes the difference.

Yes, and this is the size where it matters most. 8.7 megapixels is more than many photographs carry, so reaching it often means enlarging - and an enlarged picture prints soft however correct the pixel count is. Start from the largest original you have rather than from something already reduced.