Constant hdk::prelude::KEY_REF_BYTES

pub const KEY_REF_BYTES: usize = 32; // 0x0_000_000_000_000_020usize

Key refs are the same length as the keys themselves. The key ref is just a sha256 of the key. There are no benefits, only downsides, to having either a larger or smaller set of outputs (ref size) vs. the set of inputs (key size).