The size of A4 paper is 2 10/0mm× 297mm,

When the resolution you set is 72 pixels/inch, the image pixels of A4 paper size are 595×842.

When the resolution you set is 150 pixels/inch, the pixels of the A4 paper size image are 1240× 1754.

When the resolution you set is 300 pixels/inch, the image pixel of A4 paper size is 2479×3508.

When you choose different resolutions, the pixel size of the image will change accordingly.

Of course, don't forget 1 inch = 25.4 mm.