If I were writing that sentence I'd use, "500-word article" because the "500" and "word" are used together to describe the article. I may be overlooking some exception to this grammar rule; but, in general, when two words are used to describe a noun (and when those two words need to be together because they "work" together to describe the noun) the two words are hyphenated.
In comparison, another sentence might be, "I wrote a long, serious, article." The two words that describe the article can act alone; so the comma is used. One could say, "I wrote a long and serious article." In contrast, the "500" and "word" "need" each other and wouldn't be used alone - hence, the hyphen.