According to ADA specification of toothbrush width of brush head should be:
Length of brush head: 1-1.25 inches.
Width of brush head: 5/16-3/2 inches.
Bristles: 2-4 rows.
Tufts in a row: 5-12 tufts
5/8-3/8inches.
5/16-3/8inches.
5/8-3/16inches.
5/16-3/16inches.