That isn't too terribly small. I've seen PetSmart sell them smaller. Just make sure the little doesn't have difficulty eating the food. If they do, you can try soaking them before feeding them to him or crushing them up first. Also keep in mind that their stomachs are the size of their eye. Overfeeding can lead to problems with their swim bladder.
When you can, I suggest the Prime and getting a "treat" to vary the diet. I bought a small bottle of Prime months ago and have 6 tanks with plenty of the bottle left.
Bettas need a varied diet. What you have should be fine for now so long as the little one can eat it. When you can, frozen foods as treats are good. I like to keep frozen daphnia on hand just in case any of mine get constipated because they work as a kind of natural laxative. They also enjoy bloodworms and mosquito larvae.