think this way: you are going to return the chunk to your user as 'proof'.

the size therefore depends of your content style.

E.g. for a HN discussion, I would go 'paragraph' of each comment.

in a contract, each clause.

in a non-fiction book, maybe each section...

you can also decide to do some kind of reverse adaptive tree: you chunk at sentences level, then compare, if 'close enough', you merge them into a bigger chunk