Chino, Thank you for your recent purchase. Toilet seats come in two standard sizes: Round and Elongated. Round size is about 16.5 inches long, while Elongated size is about 2 inches longer at 18.5 inches. To measure, you typically measure the bowl to determine the size. Once you know the size, you should feel confident that a seat of the same size will fit.
Home Depot has a nice size guide online. Click on "more info" and then "instructions / assembly".
You want to measure the bowl form the center of the bolt holes to the front of the bowl. Any given bowl can be give/take a fraction of an inch but they should be close to 16.5 or 18.5 inches. If the bowl doesn't have a standard shape, you will need a specific seat that is made for that bowl. For that you'll need to know the make and model of the bowl to get the best fit. Kindest Regards, Bemis Customer Service