I don't have field experience but,
If you have read enough about MERGE table (read it from the reference manual), you should not be asking these questions.
1. How safe/stable are they?
Depends to the underlying MyISAM.
2. what is the max size for MERGE tables?
There is potentially no max size, probably limits by your available disk space
3. what is the disk size of MERGE tables (in addidtion to the traditional tables)?
Very small, it only needs to store the list of MyISAM tables name in the MERGE table in a text file, in addition of a .frm file
4. any known problems/issues/bugs?
Maybe you can check the bugs list for any known bugs
As for MERGE problem and issue, index is not used very efficiently since there is no 1 big index for all the merged table, have to search all the small index of each individual merged tables.
I believe these answers can be found in the
http://dev.mysql.com/doc