There is no fixed limit. In fact, the limits are defined not so much by
storage (or file size), but by processing (or computing time).
If you have a lot of tiddlers, macros cycling through all tiddlers (e.g.
the timeline, or list of all tags), will take longer to process.
Take a look at this page on the community wiki:
http://www.tiddlywiki.org/wiki/Performance_Issues
HTH.
-- F.