Everybody knows small aperture means strong diffraction, but there is no limit from the pixel size and density.

According toDigital SLR Camera Reviews here, so-called DLA (Diffraction Limited Aperture) is the aperture where diffraction begins to visibly affect image sharpness at the pixel level. It says as sensor pixel density increases, the narrowest aperture we can use to get perfectly pixel sharp images gets wider.

This theory assumes the image is a perfect dot and nothing else, and compares its airy disk with pixel size. If the airy disk is larger then pixel size, then the diffraction is visible.
<p class="last"]But actually, the photo image is not a dot, but a continuous surface. When diffraction occurs, the whole the image, every part, every dot gets some kind of blur. So each pixel is definitely affected by some degree of diffraction. There&rsquo;s no such thing called perfectly pixel sharp images. Diffraction could be visible at any aperture, any pixel size theoretically. The size of pixel doesn&rsquo;t limit anything. Digital sensor and film have no difference here.