It depends. I use the New Life Spectrum pellets for small fish. Those pellets are 1/3 the size of the Hikari pellets. If you can find the New Life Spectrum formula for Bettas, then the pellets will be about the size of Hikari pellets. I went with the small fish formula because I couldn't find the type made just for bettas and the pellets for medium sized fish is a little larger than Hikari pellets. Maybe a good sized King betta could eat the medium fish pellets, but I didn't want to try with regular sized betta.
Since the NLS pellets are less than half the size of the HIkari pellets, I give them 6 pellets twice a day. I've been watching them for bloating and checking the sand for uneaten food to make sure I'm not over feeding them. So far it looks like they eat all the food and aren't bloating, but I do see more poop. Since I'm wondering if I should cut back to 5 NLS pellets twice a day, I wouldn't give a King betta more than 10 pellets twice a day.
I couldn't find any in a store, so I order it on line from Petco. I woudn't bother checking Walmart or any other general store. You would have better luck at a local pet store with a good fish section. If you do have as much trouble as I did finding NLS, here's a link
to the page I brought my food from.