I don't understand why it's necessary to chunk every X characters of a document instead of chunking parts that are self-contained and individually meaningful. Luckily, essentially every document is already divided into such units, and they're called sections (or something similar) and are clearly labelled. So surely it would make more sense to summarize every section of the article (and if its sections are too big, you'd summarize sub-sections, going as many levels deep as needed and down to paragraphs if necessary).

The only drawback is that, for every corpus of documents, you would have to write custom chunking code that takes into account how big the documents' sections and sub-sections are, and how they're labelled. But this isn't realistically too demanding, and the difference in quality would probably make it absolutely worth it.