Buy this book and read it a couple times. The size of a clam does not matter one bit, but the health of the clam matters. If you get a clam that hasn't been growing and living in subpar conditions, then it's going to be an uphill battle (even for experienced keepers). 99% of vendors want to sell you what they're selling, so they'll say whatever it takes to sell you their goods/livestock. I've witnessed this several times at trade shows where the person has never kept a clam and the vendor sold them an expensive clam knowing the tank was new or the customer was new to the hobby. That's not good for the clam or the hobbyist. Back to clam size, people need to stop repeating things they've read or heard from others without confirming the information for themselves. This hobby is plagued with this regurgitation syndrome and it doesn't help anyone. If you don't know it for a fact, then don't say it or at least preface the statement with "I'm not sure if this is true or not, but I heard...." at least then the reader can decide if they should trust the info or do more research for themselves. This is especially true with clam care. Size does not matter, they don't need to be fed phyto and they need plenty of quality daylight to thrive. The amount of light is determined by the individual clam, but there are some general starting points based on species. Remember, you can have two siblings from the same spawn that are completely two different sizes that have been living side by side under the same conditions.
The easier clams to care for and start with are also the largest species, so they aren't ideal for smaller tanks. Derasa, gigas, squamosa and hippopus are generally pretty hardy and can take a wide range of light, but they are the biggest species and will require a very large tank down the road.
I treat clams like I treat acropora, they need a super stable home with lots of quality light, but acropora are very simple animals, whereas clams are complex animals with organs, so their care is a little more detailed. Look, we've all made those impulse buys, only to realize that it wasn't the smartest thing to do, but make it count and learn as much as you can and don't repeat your mistakes. If you want to keep clams, then invest into quality light and know how to keep a stable tank. If you don't know the reefing fundamentals/basics, then all of this is pointless and you'll most likely have ongoing issues. This is where most new hobbyist fail, not knowing the basics. No amount of YT videos will cover this, its experience and its knowledge passed down from others or simply reading some books on the subject. Yes, I said books, as this is too often overlooked by new reefers, books are still relevant in this hobby. In fact, one can purchase (used) volumes I & III of The Reef Aquarium and have a successful tank. Okay, I'm done screaming at the clouds, but for those reading this, learn the basics and take your time, you owe it to the animals you're trying to keep.
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