Is it better to stand or sit when eating? If it is better to sit, what posture should I go for?
There is some research that has been done on this, as well as some more anecdotal/lifestyle based evidence.
There is a study done in 1914, published in the American Journal of Physiology, where they examined the carbon dioxide tension of the blood after eating. CO2 tension is a byproduct of food metabolism, and in high amounts can cause dyspnea, acidosis and possibly altered consciousness. The findings showed that standing, then sitting, then lying each produced higher CO2 tension than the previous position. Trendelenburg (Lying with head lower than legs) produced less than lying, approximating the sitting position. Sitting erect also had lower tension than sitting reclined.
More anecdotally, this website posting looks at the traditional Indian position of sitting on the floor while eating, which promotes a more erect sitting stance than sitting on a chair/couch or reclining while eating. While not backed up with studies, it also claims that the vagus nerve response is enhanced in this position, resulting in quicker time to satiety (You eat less as a result). Many Middle Eastern cultures eat this way, as do more traditional Asian homes where the seats are merely cushions on a floor around a short table. There are many other benefits listed in the article, but for brevity and provability I won't cite them all here.
I also ran across references (But was unable to find the actual study) to a 2007 study involving women published in the Journal of the Academy of Nutrition and Dietetics, that found people standing up tended to pay less attention to the food, and as a result ate faster and more. It's fairly well established that the vagus nerve response to feel full is in the neighborhood of 20 minutes to take effect, so if you eat slower (as promoted by sitting down) you will also tend to eat less overall. It's also been shown that eating without distractions such as television and reading promotes lower consumption overall.
So in effect, it appears that sitting in an erect position is possibly the best happy medium, and if possible, eating without distractions to be able to concentrate on the food. Especially if you suffer from acid upsets or other related ailments, the more erect you are the more you may ease your symptoms.
When we sit we eat slower and digest the food more properly, because we're more calm. When standing we do things faster, hence we are less focused what we eat and not chew food properly by adding saliva and its digestive enzymes which helps to digest and metabolise your food more efficiently. More stress could also have negative effect on your esophagus. However studies from 1994 and 2010 found no relationship with the frequency of indigestion.
People with reflux symptoms are advised to remain upright during and after eating which can keep the acid down in the stomach where it belongs (so doesn't comes back up into the oesophagus).
People who have weakness of the muscles around the mouth, they should seek advice from a speech and language therapist concerning the best sitting position for safe swallowing.(NHS)
So as long as you're healthy and chew food properly in non-stressful environment, there doesn’t seem to be any harm in what position do you eat and how fast.