The Apple Smalltalk mentioned in this thread came with a full image that had to be stitched together on a hard disk before it could be used and needed a 1MB Mac and also a reduced image that could run directly from a floppy disk on a 512KB Mac. It was very usable, though reduced.
Digitalk was created from the ground up to run on 512KB PCs. For Squeak 2.2, Dan Ingalls created a reduced image of only 585KB but which included extra information so it could be used without the .sources file that is normally needed for debugging. You can try it in your browser using SqueakJS.