Constant MAX_BLOCK_SIZE 

pub const MAX_BLOCK_SIZE: u32 = _; // 131_072u32

Largest block the format allows (RFC 8878 3.1.1.2.3, 128 KiB): the single source of truth for every surface that has to name it, the C ABI included. A frame’s own maximum is the smaller of this and its window. While the spec limits block size to 128KB, the implementation uses 128kibibytes

https://github.com/facebook/zstd/blob/eca205fc7849a61ab287492931a04960ac58e031/doc/educational_decoder/zstd_decompress.c#L28-L29