Whenever I sell original artwork (or prints), I include a short written statement with the piece (attached to the back) that explains that purchase of original artwork does not confer rights to reproduce the image. While we illustrators know this is true, many buyers do not. I believe it helps avoid confusion later on to do a little education, up-front.
In your contract, as Liz said, you want to be specific. You also want to follow the description of rights being granted with the all-important "exclusionary language", e.g., "all other rights remain with the artist".
I also agree with Brit: you can request any size restrictions you please on the use of the image. It's your work, you own the rights to it. You can sell or keep whatever rights you wish.