Constant MAX_SIG_SIZE 

pub const MAX_SIG_SIZE: usize = _; // 8_192usize

The maximum size of the serialized header signature (8 KiB).

A serialized header signature holds a single signature plus its policy, which comfortably fits in a few KiB. Bounding the serialized header-signature length to a sane maximum before allocation mirrors the MAX_HEADER_SIZE check applied to the preamble.