So... This is what happened when my roommate mentioned we were allowed to paint the walls in our apartment.
Definitely the biggest art project I've worked on - the painting's 6 feet tall by 7 feet wide. Started about a month ago, been putting in an hour or two most days since then. Took a little longer than I expected, but had a a lot of fun doing it, and now I have Twilight in my dining room!
Go for it! It's pretty time consuming but it's fun. Once I found the picture I wanted to paint, I printed it out with a grid over it. Then I cut out a bunch of string and taped it to the wall to make a larger grid (you could also just draw the grid on the wall, either way). It's a lot easier to get the proportions right if you use a grid and just draw one box at a time. After that it's just a matter of finding the right colors and painting it in.