Does this mean 20 centimeters long? This sentence is missing something in English. Is something implied in Portuguese? I asked a similar question about a building height in another sentence, but this seems different, b/c the meaning here is really ambiguous -- 20 cm length or width could both work here.
The word "tem" (have) typically translates into English as "is", as does Spanish. "Eu tem cinco anos" says that I have five years but in English becomes, "I am five years (old)." Spanish people learning English will often say, "I have five years." The sentence they give includes "long" at the end, which I agree is implied as we often are refering the the length, but is not a certainty because maybe we are considering the width, not the length. Some leaves have an awesome width and this might be the focus.