Learn English – Difference between “valued” and “valuable”

When you are denoting size, there is little difference between "little" and "small". In your example sentence, either would work just fine (and mean just about the same thing in everyday usage).

The most significant difference between the two comes into play when you're describing quantity. As others have noted, only "little" can be used to describe quantities.

Another bit about "little" is that depending on whether you have the article "a" in front of it, the meaning flips:

I have a little experience teaching English.

I have little experience teaching English.

The two sentences have opposite meanings. The former indicates your having some experience, whereas the latter indicates your having no experience. Another example of the latter is the word's usage in the first paragraph, first sentence of this answer.

The word "small" has no such issues.

The term "good" in this sense is kind of a squishy term. A commodity is simply something that can be bought and sold. Futures contracts are commodities, as are financial instruments which may be abstract in the extreme. Often these are called products. The Gross Domestic Product includes goods and services. But goods are definitely distinct from services. Whoever said commodities are goods is probably restricting the definition to to material things, but that's a distinction of convenience, not an absolute definition.