When starting out, I was amazed at the lack of modern and minimalist posters for the modern teacher. My classroom rules are simple in middle school:
Work Hard & Be Nice.
Fight Evil. Read Books.
These will print up to 18x24. And if you are looking for a budget print, they make a great Engineering Print from your local print shop( An 18x24 will run you about $3-5, in black and white).