Every periodical will tell you in their guidelines the word count they prefer. Typically it is given in a range, such as 800-1,500 words. When determining the appropriate length for the piece you are offering. If it is a feature article or an article of prime importance to the readers, then offer it at the longer length. If it's more of a supplementary piece, offer it at the shorter length. Book publishers will also indicate length preferences, usually longer for fiction than for nonfiction. Again, check their guidelines.
Some writers set a goal for a certain word count per day to keep them moving ahead on their current project. A daily goal of any number of words will help you meet your ultimate word count goal for the project. Here are some tips to help you stick to the appropriate word count: http://writetodone.com/?s=sticking+to+your+word+count.