Chiles come in all shapes and sizes but in Saigon these days, the predominant ones are largish hot ones that resemble long chiles. They have heat and are what you'll find sliced and served at the table. They don't have the perfume heat of smaller ot hiem but do pack a punch.
Last year, when I visited, these chiles weren't everywhere and you could sometimes get smaller ones. Looks like these are the ones people like nowadays. They keep better once cut and definitely have more zip than your jalapeno in the States!
Compare the size of the chiles to the limes, which are regular sized ones. The chiles are a good 3 to 4 inches long. At markets I found the chiles already ground up so that you don't have to chop them yourself. Usually, there's a vendor who sells pre-chopped lemongrass, garlic and chiles. Other aromatics, such as shallots and lemongrass are typically sold by the same person too. The chopping is done by machine. Quite convenient.
At casual restaurants (joints), a small container of pre-chopped chiles sits at the table, ready to be added to sauces, etc. There's no Sriracha or chile garlic sauce here! You could certainly use these chiles to make your own chile garlic sauce.