You do NOT need a special Tunisian crochet hook for this pattern. The pattern is worked in small diamonds which are joined as you go along according to a chart in mitred Tunisian crochet squares. The small size of hottie needs a 4.0mm crochet hook, the large (standard) size is worked with a 4.5mm crochet hood. For both the neck is worked in knitted rib on double pointed needles.

TENSION:
SMALL SIZE 1 diamond measures 7cm (2.75 inches) from point to point, 6cm (2.5inches) along each edge and 6cm (2.5inchest) high, before attaching and worked on a 4.0mm hook.
AVERAGE SIZE 1 diamond measures 8cm (3.125inches) from point to point, or 7cm(2.75 inches) along each edge and 7cm (2.75 inches) high before attaching and worked on a 4.5mm hook

Photo is of small size