How many words is a typical scientific publication (particularly in biology)?
This source mentions that they're "typically 3,000 to 10,000 words in length". This page may give you some tips on the length of specific parts of the paper, as well.

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. ;-)