Debian Jr. was first released in Debian 3.0 "Woody". Therefore, to run it, you must at least have a Debian Woody system.
Debian Jr. will add approximately 320M to a base Woody install. In addition to this, some space is required for X, a window manager, and some other programs of your choosing, as well as space for home directories and system overhead. The recommended minimum hard drive size is therefore 700M, with 1G being preferred. Of course, smaller installations are possible if you choose fewer components to install.
Debian Jr. Quick Guide23 March 2003