Methods, systems and apparatus, including computer program products, for processing digital data. An approximate storage size is predicted for an output representation that uses variable length encoding. The approximate storage size can be used to determine one or more compression parameters to satisfy a global constraint, such as a maximum storage size for a compressed representation of the digital data. In a user interface, storage sizes can be graphically represented for multiple images. In the graphical representation, the storage size is represented for each image by a corresponding graphics object that includes a visual representation of the image and has a linear size that is proportional to the storage size of that image.