It depends on your level of skill as DJ points out.
The old measure was 6" above head height for surfing. As boards got smaller and moves became more aggressive lengths dropped to assist with changing hands and the use of the paddle in hard turns.
I run mine at 4" above. However I don't ride small boards and I am too old for radical turns. As Sweeper says you can always cut it down.
Watch other SUPsurfers, talk to other SUPsurfers and experiment till you find what suits you and your style of surfing.
There is no "one size fits all" measure.
ET.