Fence gap (gap in knowledge)

[PerranOak]

I lost some featherboard fencing and am left with a gap of 2725mm. I guess I need to put a post in between the existing two - both are very stable and not rotten and are 100mm square. So, a few questions:
Do I need a post of the same size for this gap or will a smaller one do?
How far down does it have to sit?
How do I best attach the cross members to the posts?
Thank you.

 

[Deluks]

You can get 3m arris rails which should bridge the gap without another post.
Sit a small 2x2 post in the centre of the bottom arris rail, sit it on a a half buried brick.

A gravel board should be used.
You can use a timber gravel board or a 3m concrete one.

 

[foxhole]

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