Constant MAX_PRODUCT_MESSAGE_BYTES 

pub const MAX_PRODUCT_MESSAGE_BYTES: usize = crate::MAX_NIP44_WRITER_PLAINTEXT_BYTES; // 65_535usize

Maximum UTF-8 bytes in a portable message draft.

This is only the cheap single-field ceiling. Before any durable or platform effect, the SDK measures the complete rumor, seal, and wrapper and may reject a smaller draft whose tags consume the remaining wire budget.