If you want to split for two staves I go for 4-5" , can go as low as 3", as that will give you about 3.5 mid stave which still gives a fairly wide bow if it's a 'white wood'.
You can even go as low as 2" min at the small end and make a single simple bow.
Depends a bit on the wood too, if you have to remove any sapwood then you need bigger.
It's always a trade off too small you don't have enough wood, too big, it weighs a ton and you are chopping/sawing away endless waste.
If you want to split/saw into
quarters then you want 5"plus.
BTW, is it just me or do you find that when you go back for that 'great piece of timber'
it's never quite so good second time of looking
Del