Problem is solved.
I have legacy version of the app in windows 6.xxx
This is how it went:
1. Locate all oversized notes by sorting and export them individually to enex files.
2. Delete the attatchments in those notes mentioned in pt. 1
3. Tag all notes in each individual notebook with "NB_notebookname" to retain notebook struckture
4. Tag all notes in each individual sub-notebook with "SNB_notebookname" to retain sub-notebook struckture
5. Export all notes to one big enex file.
6. Import enex into joplin
7. Recreate struckture by filtering by "tags" and moving into new Notebooks and then new Sub-notebooks
8. I'm free
