Welcome to the boards.

It's very hard to tell with petstore bettas. They range in age. Size isn't a good measurement for age. A smaller betta may not have gotten as much food as the others or didn't have as clean water. You should always purchase a healthy fish... regardless of size or color. If there are a lot of healthy bettas then you can choose your color from the healthy ones. Some fish may already be too gone to rescue. The one you had previously may have been one of the ones too gone already.

Your new betta may be able to grow bigger, but I wouldn't force it by giving him way too much food. You should only be feeding once a day as much as your betta can consume in 2 minutes. It may seem like he wants to be fed more than that, but trust me... once a day is plenty. Since you've been researching, I wanted to give you a reputable website you can go to for information.
Betta Talk

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Phishie
Site Coordinator

"An ounce of prevention is worth a pound of cure."