Design is a very creative field, but it also uses math, logic, and reason to aid in designing. For example, a flyer, advertisement, banner, poster, etc. will all be designed within the dimensions of a page. A common dimension for a flyer would be 8.5″ x 11″ (8.5 inches wide and 11 inches tall) This is the common size of a sheet of paper. Print advertisements and posters are more likely to be in custom sizes that require a more specialized printer.
You can also have dimensions of design in terms of pixels. Pixel sizing is used when you are creating a design that will be displayed on smartwatch, phone, tablet, TV, and computer screens.
Sizing and math is important in terms of the photos you use in your design, the font size, and the sizes of shapes. From small to large sizing there are different challenges you will face with your designs.
Designing on small dimensions means that you will have to make sure your design can still be read and noticed. You may have to have less elements in your design, because there is not as much space to work with. Larger designs mean that the elements must be much larger. You also have to take into consideration how close the viewer will be when looking at your design. If they are going to be looking at it from close up (1-2 feet away) you can get away with making the text very small. If they are going to be looking at it from a far distance and for not very long (for example a billboard on the side of the road) you will need to make the elements very large and easy to take in at a glance.
Sizing is also very important to consistency in your design. You must develop a method to the sizing of your type, shapes, and images. For example you may have a specific size to use for the main heading, sub headings, body copy, and captions within a single design that you will want to stick to.
If you want an element of your design like an image to take up 1/3, 1/4, 1/2, 2/3, or 3/4 of your design space, you can figure this out easily by taking the total length or width of your design and applying the fraction to it. For example, if a graphic you are making is 400 pixels tall and you want the image to take up 3/4 of the space, take 400 divided by 4 (100) and than multiply that number by 3 (300). So to for the image to take up 3/4 of the space vertically it would need to be 300 pixels tall.
This is how you can use a basic understanding of math to help aid you in your designs.