It depends on the frame size you choose. Every frame requires a few bytes of additional metadata, how much exactly depends on other compression settings (e.g. frame checksums, which are 4 byte, are only present if enabled). I just tested with a 1G file and compression level 3. zstd compresses it to 559M, zeekstd with a 2M frame to 565M. If I increase the frame size to 4M, zeekstd yields 562M.

I will add a section to the readme, this is a good question that other people might have too!

It depends on content and compression options. ZSTD has four different compression methods: Raw literals, RLE literals, Compressed literals and Treeless literals. I assume that the last two might suffer the most if content is splitted.