There is no defined word limit; an excerpt more than a few words does not constitute plagiarism if it is properly cited, nor does the length matter for the purpose of defining quoting as plagiarism.
Make sure you keep direct citations minimal, they should not exceed 10% of the total volume of the writing and follow respective formatting regulations for longer quotations, such as block quotations for 40+ words in APA style and for longer than four lines in MLA. Please note that excessive quoting dilutes your own argument, so paraphrase whenever possible and check for plagiarism.