Short version: you are probably looking at a giant, soft sand-dollar that moves by rhythmic waves of contraction on its belly. It would eat plants mostly by either sliding over them and eating them with lots of small mouths on its belly or one extremely wide but narrow mouth with lots of tentacles or similar things that constantly shovel plants into it. It would likely have a lot of holes on its back leading to a large number of individual lungs.
There are a few things you need to take into account:
- Weight distribution - the weight needs to be spread out, both where the organism touches the ground and within its own body.
- Momentum - the more mass, the harder it is to change the way you are moving. Roughly speaking, muscle strength grows with the square of body length, while mass grows with the cube of body length. So if the body length increases by ten times, strength will grow 100 times while mass will grow 1000 times. So moving around and changing direction become proportionally harder as body size increases.
- Nutrient and waste transfer - things like oxygen and food need to get all parts of the body, and waste needs to get collected from all parts of the body. This will be harder because, again, any opening of the body will grow with the square of length but the amount of tissue that opening has to serve will grow with the cube of length.
- Control - it takes time for signals to travel through the body. The bigger the body, the longer it takes. This means larger animals will necessarily have lower response times. A larger animal also means that there is more the brain has to control. Since limbs need to be controlled, more limbs are harder to control than fewer limbs. The more joints a limb has, the harder it is to control, etc.
- Feeding - there is a reason the biggest animals in history have been herbivores: it is a lot easier to get a lot of plant matter than animal matter. This comes down to the 10% law, which is a rough rule of thumb that states that each level in a food chain must be 1/10th the total size of the one below it.
So what we are looking at overall will be a very slow-moving organism with slow reaction times that needs a lot of nutrients and has trouble getting enough oxygen to its body. Which is consistent with your heterotroph idea. Edit I got hetetroph and autotroph mixed up. Comments set me straight.
Next lets look at body shape. You might think that a long body would be good to spread out the weight. But this is an area where the communication and nutrient transfer issues come into play. The longer the organism, the longer it takes for control signals and nutrients to get from where they start to where they are needed. So I think the most efficient body form would be disc-shaped, as short as possible vertically but spread out in a circle along the ground.
For locomotion, you might thing having lots of small legs would be the best solution. However, the more legs there are the more brain power is needed to control each one. So I think it would be best to have no legs at all. The best approach would be one that requires no vertical movement at all, so probably the animal would move by rhythmically stretching and contracting the muscles on its belly, like a snake or slug, creating waves of movement that carry it along. This would allow the weight to be distributed across the body's entire lower surface, and would require less direct brain control since the motions could be regulated by fairly simply brain circuits.
For feeding, nutrient transfer becomes a problem. If you are really flexible, the most efficient approach would probably be a bunch of jaws scattered over the lower surface of the body that eat whatever it comes across as it moves. Each could have a separate digestive system that provides food to areas of the body nearby.
If you want to keep to the standard bilateral body plan like most animals on Earth have, that means one mouth at one end. The mouth would probably then by pretty wide, taking up a large portion of the front end of the animal, probably with lots of small tongues, trunks, tentacles, or other sorts of protrusions from the front that mostly automatically grab anything they touch and shovel it into the mouth. This would again require less brain power, since the brain would mostly just need to tell them to start or stop. The intestines would probably be long and spread out throughout the body. Each given part of the digestive system would only pull out a small portion of the nutrients, but since it is spread throughout much of the body nutrients from the digestive system wouldn't have to travel far to reach every part of the body.
Breathing will be an even bigger issue. Breathing through the skin is right out, again since the amount of skin grows with the square of body length while the amount of tissue grows with the cube. Some sort of active breathing apparatus like a lung is necessary. But an individual lung would require an infeasibly wide "throat" to feed it, and would likely get crushed under the animals' shear weight. So it would almost certainly need a bunch of breathing tubes spread over its back, each leading to a relatively small lung close to the surface to minimize the amount of weight pressing down on it.
If the animal mostly depends on its larger size for protection, the brain would probably be somewhere between the center of the body and the front edge, closer to the center, with lots of smaller ganglia (nerver clusters) scattered evenly throughout the body to handle converting overall commands (like "move sidewise") to specific muscle commands (expand and contract this patch of skin at this rate). This would minimize the amount of time needed for signals to go from the brain to other parts of the body.
If the animal, on the other hand, has to actively defend itself, then the brain would probably be close to the front, with whatever senses and defenses are relevant being closer to the brain. There would probably be fewer, larger ganglia, with extremely high-speed dedicated neural connections between each ganglion and the brain itself, allowing some limited degree of coordinated response to attacks from the side.