Chunking strategy for governments and internal org docs

A developer seeking advice on building a retrieval-augmented generation (RAG) system for government and internal organizational documents asks about optimal chunk sizes for BM25 and semantic search (cosine similarity) retrieval, and what data should be stored in a relational database (PostgreSQL) versus a vector database (OpenSearch or Milvus) for efficient retrieval. The query also requests recommendations for research papers or codebases to reference.