To introduce you to this concept lets use a high school score board. In this example, pretend each light bulbs represents an individual pixel on a computer screen. In reality, each of the bulbs displayed here is defined by multiple pixels which is why you can see them so easy. Look at the numbers displayed under "TOL" on the lower right corner of the board. Here you can see it takes a grid of 3 bulbs x 5 bulbs in order to display the numbers 1-9. This is important to understand when working on objects such as a licence plate. Sometimes the plate is simply defined by to few pixels. Once you consider how many numbers and letters you have in a plate, the orientation of the plate, lighting, and compression, it is more likely you would need something in the ball park of 25Hx50W pixels.