I didn't measure it, but I'd guess just under 1cm based on full-size hornworms. The horn on this one only looks massive since the caterpillar is so young. The caterpillar will grow, but the horn will remain about the same length.
Just some small guys: Olive-Shaded Bird-Dropping Moth Spoiler Pale Gray Bird-Dropping Moth or Schlaeger's Fruitworm Moth (impossible to distinguish by photos) Spoiler Labyrinth Moth Spoiler Yellow-Winged Oak Leafroller Moth Spoiler
