Generally most TBX files are pretty small—a few MB or less. There is no hard limit to size to a practical limit will be resources getting maxed out either in the app or the host OS.
Some of the ideas behind Tinderbox do have limitations at large scale. Spatial hypertext works well with dozens or hundreds of notes. If you have millions of notes, it’s harder to make any sense of the map, and it’s hard to know where a new note ought to go. Hyperbolic view can likewise become very busy with large numbers of notes.
Adding images to notes (i.e. to their $Text) or as image adornments on maps will have far greater effect on document size than ordinary textual content (in $Text or as attribute values). Occasionally, users will discover hard limits by building documents larger than anyone had tried to build before but there is no fixed limit.
As a general guideline, Tinderbox is comfortable with hundreds or a few thousand notes. If needing millions of notes, a database application is really the correct. In between those sizes is a grey area. If TBXs are operating smoothly, size should not be a cause for concern.