Stirrup size for posts

Hi, was wanting to cement in stirrups into the ground for 1.8 mtr tall merbau posts for a screen I'm building, can anyone help me with the length stirrup I would need, I've already looked up how to cement it in and to leave 100mm above ground but couldn't find how much I would need under ground to support the structure properly. The posts are
90x90 mm in diameter if this helps. Thanks.


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Stirrup size for posts