Experience suggests a 35mm negative can be printed to about 15" on the long side, before grain becomes noticeable. Depending on film, developer, processing and to some extent, paper. Older 35mm lenses began to give up on absolute resolution at a similar size, but grain gives the illusion of sharpness. Which is why printers add extra grain to soft photographs. Given a sufficiently sharp enlarging lens and viewing distance, a small negative can be printed to any size, and appear sharp.
Modern lenses are very sharp by comparison, sometimes aided by in camera processing. With a high megapixel digital camera and a modern lens, an image can be printed much larger without any hint of texture. A clear blue sky in digital monochrome will print as solid grey, with only a change in density near the zenith. That may work aesthetically in counterpoint to a highly textured subject. More often it will be dead space, devoid of interest. A black and white photo is necessarily an abstraction, for which grain or noise provides a key for the eye to latch onto. Warm or cool toning fulfils a similar role, adding abstract qualities to an image based in reality.