Modern United States copyright law protects literary works, musical works, dramatic works, pantomimes and choreographic works, pictorial, graphic, and sculptural works, motion pictures and other audiovisual works, sound recordings, and architectural works (17 U.S.C. § 102(a)). Many such works may be found in your local museum. Copyright law further provides owners of copyright the exclusive right to reproduce, adapt, distribute, publicly perform, and publicly display their works (17 U.S.C. § 106(1)-(6)). These rights may be transferred or conveyed all together or individually. You and I are probably most interested in the exclusive rights to reproduce a work and to prepare derivative works.
The right to reproduce a work involves creating copies of that work. They need not be exact copies, but at a minimum must be “substantially similar,” which generally means “whether an average lay observer would recognize the alleged copy as having been appropriated from the copyrighted work.” As the law says, “‘Copies’ are material objects… in which a work is fixed by any method now known or later developed, and from which the work can be perceived, reproduced, or otherwise communicated, either directly or with the aid of a machine or device” (17 U.S.C. § 101). Printing, photocopying, and photographing a work are all exclusive rights of the rights-holder, meaning that just because a museum possesses a work does not mean that it may take pictures and/or print pictures of the work without appropriate permission.
Creating a 4′ x 4′ poster reproduction of a 2′ x 2′ work, differing only in size, would be a reproduction requiring the permission of the rights-holder because only the rights-holder possesses the exclusive right to prepare a derivative work. Specifically, this is the right to create new works based on another work whose rights are possessed by the rights-holder by “recasting, transforming, or adapting” the original work (17 U.S.C. § 101).
 Shapiro, M., and Brett I. Miller. A Museum Guide to Copyright and Trademark. Washington, D.C.: American Association of Museums, 1999, at p. 27