How long should your answers be?
Answer: About three-fourths the length of Lincoln's Gettysburg Address
The question of length is a good one. It's also a difficult one to answer as it depends on the question itself, your style of writing, the detail which you give to your examples, and—since this is an open-book exam where you basically could prepare questions in advance and then cut and paste from other sources (with proper credit given to those sources)—a good answer can vary quite a bit in length.
At a minimum you should have a beginning, a middle, and an end (sometimes also known as introduction, body, conclusion).
You should also be sure to answer the question(s) asked, and if there are two, three or more parts to a question, be sure to answer all of them.
Be sure to give examples when you make a statement.
I think it is a good thing to have a look at the OWL's advice before every exam:
Writing Essays for Exams
To use their recommendations, a well focused, well organized, well supported, well packaged essay answer could be done (for most of the questions) in the equivalent of about a page and a half to two "normal" pages (double-spaced, one-inch margins, #11 or #12 font)—which is about 375-500 words.
How long was the Gettysburg Address?
263 or 268 or 270 words depending on which printed version you check
A standard "letter size" 8-1/2 X 11 sheet of paper has about 255 words, with a size 12 font
So your answer should be about one page long, two if you use the Owl's recommendations