I can't recall what size needle I used, I think it was 6.5mm. I took inspiration from this Easy Chunky Cabled Scarf and heavily modified it to reach the following pattern:
Cast on 24
Knit 4 rows garter stitch
Rows 1, 3, 5, & : s1, (k1, p1) x3, k2, p6, k2, (p1, k1) x 3, k1
Rows 2, 4 and 6: s1, (p1, k1) x 3, p2, k6, p2, (k1, p1) x 3, k1
Row 8 (cable row): s1, (p1, k1) x 3, p2, c6, p2, (k1, p1) x 3, k1
Cable 6 = slip first three stitches to a cable needle, knit 3 then knit the 3 stitches from the cable knitting. Continue with pattern.
Repeat this 8-row pattern until desired length is reached.
Knit 4 rows garter stitch and bind off.
I recommend a bulky or super bulky yarn for this pattern. The ribbing on either side of the cable pulls the yarn in slightly to create quite a narrow scarf. Here is the same pattern in DK yarn.
Carnival Soft 8 Ply acrylic from Big W