General

If you want to increase your capacity, try bloating with Gatorade or something else that's electrolyte-balanced. Train gradually--you don't want to make yourself sick. If all you can handle at first is a half liter, then just stop there, practice that for like a week, and then increase your intake a little bit every week. Eventually your stomach capacity will increase.

If gaining is your goal, you don't necessarily have to stuff yourself. From what I've heard and read, grazing on little snacks all day is more efficient for putting on weight anyway. If you eat a bunch at once your body will probably "discard" what you don't need as waste, but if you just stay eating all day but don't tax yourself you're more likely to store those calories a bit at a time and you'll be chubby in no time.

If you want to make yourself hungrier so you can eat more, delay breakfast as much as you can. This has the added effect of tricking your body into thinking food is scarce so you'll store more fat.

And If you just want to see your belly bigger in a short timeframe but can't eat enough for it to make a satisfying difference before you start to feel sick...have you considered air inflation through the other end?

The little experience I had taught me one simple thing: You have to choose between a full belly or a fat belly.

If you really like the feeling of fullness then you have quite a road ahead of you to train your stomach. But most importantly, you must not push yourself no matter how good it feels. It might bring consequences that you don't want.

I had a habit of stuffing and bloating with several things all at the same time, I was just disappointed that my belly wasn't expanding as much as I wanted so I just kept doing it, air, soda etc. Just forcing foods and drinks I didn't like too (I hate carbonated drinks and alcohol).

Don't do this. I have now an ulcer that makes it painful (not in the good way) to eat. Not to mention the acid reflux.

So please BE PATIENT. don't do what I did.

If I want to feel the most stuffed for the longest amount of time, here are some pointers I use:

1. Start with bland dry and starchy. I don't mean so bland that there is no flavor, but spiciness, saltiness, tartness, acidity, and herbal flavors build up and eventually become overwhelming. Only drink enough liquid to keep things going down smoothly. Eat with pace until you get to the point where you start chipmunking or overchewing.
2. Then switch to the main course. This is actually the opposite of what competitive eaters do, because they are trying to not feel fullness. You will get a bit of a second wind for this because it is good food.
3. Chase with non-carbonated room temperature liquid. I usually will sip warm peppermint tea or a slightly cool, but not cold gatorade. Bubbles will put too much pressure on you too quickly and are more likely to make you sick. The liquid will help everything in you settle at first, and then when it hits your starchy first course, it will get soaked up and you will really feel full, but not like you ate too much too fast.
4. Take your time picking and grazing and moving around and rubbing your belly. Eventually the feeling of fullness will subside and you should keep topping off with a few bites every couple of minutes until you get to the point where all the food is gone or you don't want any more of any of it.
5. There is always room for dessert - After some time, when you do want to eat again to stay stuffed, but you don't want any more of what you've been eating, have a sweet treat.

Good luck.