The height of the functional sofa bed is preferably between 35 and 42 cm, which is approximately equal to the height of the person's calf. Too high or too low size is not conducive to the relaxation of human muscles, prolonged use can easily cause body aches. The seat depth is preferably between 48-55 cm, that is, when the back is placed against the back of the sofa as much as possible, the person's knee should still be outside the seat. From the ground up, the height of the backrest of the sofa bed should be kept between 68-72 cm. Within this range, the comfort of the human body is the strongest. In addition, the inclination of the backrest is preferably between 100 and 108 degrees, and the height of the handrail is preferably between 62 and 65 cm.