Using my yarn and hook, square was 4”.

To get to 6” replaced Rnd 10 and added 2 more rnds:

Using pattern for Crochet Flower Bud Head Band:
Rnd 10 (green): * (2 tr, ch 2, 2 tr) in corner, then work per pattern across side with beg v-st in 1st dc, rep fr * around. (6 v-st per side)
Rnd 11 (yellow): * (2 dc, ch 2, 2 dc) in corner, ch 2, work per pattern across side, ch 2, rep fr * around. (6 buds per side)
Rnd 12 (peach): * (2 sc, ch 2, 2 sc) in corner, sk 1st dc, sc in next dc, sc in ch-2 sp (before 1st bud), long hdc betw corner tr and 1st dc of v-st from Rnd 10; work per pattern across thru last bud of side, long hdc betw last dc of v-st and corner tr fr Rnd 10, sk next dc, sc in last dc, rep fr * around. (26 sts per side, 4 ch-2 corner sp)