Since most yards are large and oddly shaped it is often simpler to estimate a large area by breaking it into several smaller areas. Smaller areas are also easier to measure. Look for small sections that can be approximated as a square, rectangular, or circular area. Keep in mind that you are trying to ESTIMATE the size of the area in question. Measurements to the nearest foot or so should be a good start.
It helps to draw out a plan of your lot on paper with any buildings, driveways, sidewalks, and other non-planted areas identified. Next identify the areas you want to seed or sod and divide all areas into the largest squares, rectangles or circles possible.