There are no fixed limits, though there might be practical ones due to recursive algorithms running out of stack space while trying to traverse a very deep tree, and for lookup strings I believe the maximum size of a BSTR is 2GB so you couldn't have a lookupstring longer than that. If you require a very deep tree I recommend testing the worst case depth you expect to need to support and see if you get any errors. I would expect you to not see any problems except with very deep trees.
Hope this helps,
-Doug
