There is a difference in the size of laptops and notebooks.
From the several definitions I found when I searched google for the definition of "laptop", there is a lot of confusion. A site sponsored by Dell said that a Notebook computer is generally larger than a laptop. Another site said just the opposite.
From my own experience, the term "laptop" first came into use with the Radio Shack Model 100. At that time, to qualify as a laptop a computer needed to run on batteries and have a built in screen.
As portable computers improved, the term "notebook" was introduced. When it came out, it was used to define a laptop computer that was small enough to fit in a briefcase, like a notebook.
At frist, the notebook computers had a smaller screen than the laptops of the same generation.
For the past several years any difference that previously existed has been blurred. For all practical purposes the term "laptop" and "notebook" are used the same to refer to a computer that runs on batteries and has a built-in flat screen.