As noted, some airlines specify a maximum measurement (length + width + depth) of 36 inches. Of course has to be small enough in each dimension to fit under the seat.
Many laptop backpacks are larger than this, some by as little as 1/4 inch and some by 4 or 5 inches. FWIW, I like to use ebags.com to shop for laptop bags because their site can tell you whether a specific model of laptop computer will fit in a specific bag, or show you only bags into which it will fit.