Hi @nossair. I'm no expert on the subject but I imagine that the thicker the neck the smaller the valute. If you're neck is made using a scarf joint from a 1" blank then you may need a bigger valute than an angled headstock cut from a single piece 2" neck blank.

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Posted by: @nossair

so does the volute size matter or you need just some extra mms to make the neck stronger? 

@nossair It depends on what kind of guitar. If it’s a standard guitar I would say it’s personal preference. For me, if it’s got an angled headstock, I would have a bigger volute, it will give it more strength. I always picture all of those broken headstocks on Gibsons that need repairing. They have quite steep angled headstocks and no volute really, it’s a natural weak point I think. I think it’s important to have one but the size is up to you. 
I also think that if you are building a guitar that is out of the ordinary, such as a 7, 8, 9 string or something, an appropriately sized volute should be included. I think a volute will add to the stability of the headstock and neck, not just strength. 
When the tension of the strings are pulling the neck forwards, a decent volute can only add a bit of stability to the whole thing. It depends on what gauge strings are used, if heavier ones are used, there will be more tension on the neck. 
Of course, this is my thought process and I may be talking rubbish but that is how I would go about it. 

Hope this helps. 👍 
Boo

Thank you. Now I understand more.

Wow..thank you for the explain...