Function auto_block 

pub fn auto_block(len: u64) -> u64

Pick a BlockCacheReader block size from the file length: bigger files get bigger blocks so a remote query makes far fewer (but larger) round trips — 128 KiB ≤ 10 MB, 256 KiB ≤ 100 MB, 512 KiB above. The over-fetch is modest next to the round-trip latency it saves on a high-latency link (S3/CDN). Shared by the CLI and the wasm client so both size identically; the file length is known for free from the opening HEAD / Content-Range.