A 10-12 inch laptop have very thin screens are also very light to carry around. But this portability comes at the price of a smaller keyboard size. A lot of laptops in the size range nowadays can also be used as tablets. A 13-14 inch laptop provides the best balance of usability and portability. The laptops usually weight around 1.5-2 kilos.
15-inch screens are the most popular of the lot and are usually the least expensive. However, they are quite bulky. A 17-18 inch laptop meanwhile is primarily meant for the gamers. However, it can also be used for work related activities.