The current limit on part size is currently 2048x2048x2048 studs, but I believe we should be able to make a part’s size at least quadruple that, if not unlimited. It doesn’t have many use cases, but the few it does have are big ones. Here’s one of the big ones I noticed:
The current terrain tool’s Region editor uses an invisible part with a SelectionBox to control regions. There is a cap on regions due to part size, and upping the clamp on part size would allow for bigger regions in the Terrain editor.
If this were added, more use cases would most likely start to pop up.