I've heard anywhere from 2,000 - 10,000 words is typical for a publication in an average journal. What's your experience?
BTW, I think the length of the publication can also be dependent on where you are submitting it. Also, you can check yourself creatively by going to sites like http://journals.plos.org/plosbiology/, and copy+paste the text into a word counter. ;-)
My experiences with "average" journals (by which I mean that they aren't extremely high-end like Nature or Science) is that typically a publication is around 4,500-7,500 words.
My old adviser used to say that if his manuscript reached 30-40 pages (typewritten, Times New Roman, size 12, double-spaced), he would split it into two publications instead. I wouldn't suggest this method because it's a lot of work to write one manuscript, and this way you would have to go through many more drafting stages to get two manuscripts that were thorough, told the complete story, and yet did not overlap.
A typical publication in the areas of biology that I work in, is like an iceberg. The "paper" per se is likely to be only a couple thousand words (for example Nature articles are only 3000 words long, and a number of other high profile publications also have tight limits). That small portion sticking above the surface is typically backed by anywhere from 10 to 50 pages of supplementary material, which contains the bulk of the paper.