A lens hood is to block direct light from a source (,like the sun and light bulbs,) creating glare on the different surfaces of the glass on the lens. Think of it like putting your hand over your forehead to block sun glare. Most lens hoods thread onto the end of the lens. This is the same way to apply filters to the lens. If you have filters, the size in mm will be marked.
Different models of cameras have different lenses designed to fit it. Even more, other companies make lenses to fit Cannon cameras. They could have different filter sizes and all be a 50 mm lens. You could take your camera to a store and try filters to find the thread size. You might could find a listing in some magazine like Shutterbug or some literature from the manufacturer of the lens that offered filters, listing the thread size.
When getting a lens hood, be careful about vignetting. You don't want part of the hood to show up in the image. A wider angle, like the 18-55 at 18 will need a wider, shorter one to keep out of the picture. The 75-300 at 300 selects a narrow picture area and a narrower, longer one lets you point more toward the light source w/o glare.
What might be confusing you is that the lens size. A 50mm lens has a focal length of 50mm, which covers on a 35mm film frame to be close to our normal vision. The outside opening of the lens might measure 49mm, 52mm, 55mm, or something else.