1.)Check in the manual that manufacture has given when you bought laptop.
2.)Turn backside of the laptop and look for specifications.
3.) Mostly in top right area of screen, you may find it along with number.
For eg: My desktop monitor has "H243HX" that is 24 inch screen size.
4.) Check in the official site with exact model name of your laptop.
5.) Type in google or any search engine with "screensize laptop ".