I would say it depends more on your experience level, and what your plans are for the particular fish. Per this interview, with a well known and accomplished show breeder, it is more than adequate.
"Phil: I use beanie baby containers that hold 1/2 gallon of water. These must be 100% completely changed every third day."
Per the Ebay link;
"This tank is suitable for only 1 Male Betta / Siamese fighter, 25% of the water should be changed at least twice to three times a week."
While they are on the money with stocking I would suggest larger water changes than 25%. Jars ranging from 1/2 to 1 gallon are commonly used by breeders, as long as you are mindful of the maintenance requirements it isn't a problem.