cometbft: `block::Size::max_bytes` should be `i64`

[melekes]

The documentation for the 'max_bytes' field states that its value must be >= -1; however, since the field is of type u64, it cannot represent negative values. Consider revising the comment to reflect the valid range (e.g. 0 or a specific special value) or adjust the field type accordingly.

Originally posted by @Copilot in #82 (comment)