Generally, the organ that covers the most surface area is the skin or cutaneous membrane. This of course is on the exterior of the body. Large internal organs include the liver which has a large mass and can weigh several pounds. The diseased liver often is even larger secondary to hepatomegaly. The intestines are also quite large, the stretched out bowels measure between 22 and 26 ft. but have a relatively small circumference. The human heart is somewhat of a large organ, on average it is the approximate size of the closed fist, this varies with age and general body stature. The spleen is also large in size, the average adult spleen can hold in reserve approximately 400 to 600 ml of blood. In length, the organs of the cardiovascular system ( arteries and veins) measure many, many meters.
The typical answer to this question is that the largest organ of the human body is the skin. So if you are talking about the whole body (and not the organs that are internal), then the answer is the skin.
When it comes to internal organs of the body, the largest of these is the liver. But it is not nearly as large as the skin and everything I have ever read names the skin as the biggest organ of the body.
Please follow the two links below to look at some information about the skin and the liver.