A mid-passage change-clef is engraved smaller than a system-opening clef

A clef sign appearing mid-system (a change clef) is drawn at a visibly reduced size relative to the full-size clef that opens each system — conventionally around two-thirds to three-quarters of the normal clef glyph size — signaling to the reader that this is a same-passage change rather than a new system's opening clef.

A mid-passage clef change rendered at full system-opening size, which reads ambiguously as though a new system had begun, or is simply visually jarring against the surrounding full-size notation.

Gould, Behind Bars, Clefs chapter, states the reduced-size convention explicitly. Ross, The Art of Music Engraving, agrees and gives the traditional engraver's proportion (roughly 2/3 to 3/4 scale). Read, Music Notation, agrees. SMuFL encodes this via a distinct 'small clef' glyph variant, and Dorico/Finale/LilyPond all implement a reduced-size change-clef by default.

This rule is about where marks land on the page, so it can only be judged after engraving. Collision and spacing faults live here.