Consider where the actual difficulty lies in each case.
1. An unusual instance under sariq: the ordinary expression is known, but classification of a case is uncertain: khafi.
2. Whether a food counts as idam: the expression's customary scope needs reflection: mushkal.
3. The technical legal extent of riba: ordinary language does not settle the required legal boundaries: an example of mujmal.
4. The exact specific meaning of disconnected letters: the book does not claim to determine it: mutashabih.
None of these classifications follows from sentence length. Each depends on the cause of uncertainty and the form of resolution.