They do not specialize in fish. Even places that do, often have tank size suggestions that experienced hobbyists know won't work. Again, bigger can't hurt. Smaller almost always will, it's just a matter of time.
Look in the Sick Fish section and see how many people have posts about their poor Tang that has Ich or HLLE
that is in a smaller than 6' tank. Then try to find someone with the same issues that has the same fish, but in a 6' or larger tank. You will find dozens of "too small" issues, but rarely the opposite.