The minumum requirement for pixel-based artwork (.tif, .eps, .jpg, .psd, etc.) is  a minimum of 266 ppi (pixels per inch) or dpi (dots per inch) at the size it will be used. This recommendation applies to photos, ads and graphics that do not contain small type. If you have type smaller than 10 pt, (or fine-line artwork) the type (lines) will be sharper if created in a vector-based design program like Illustrator or Indesign.

When an image is enlarged in a design program, the resolution is proportionally lower than the original file. For example, a 2" x 2", 300 ppi photo, at 100% will print sharply. Enlarged to 200% (4" x 4"), the image is reduced to 150 ppi, which is too low-resolution and will print pixelated and/or blurry. Here is an exaggerated visual: