I use a Z splice these days, I have done Vs and I have had one fail (managed to repair it ) I'd go as long as possible depending on how much stiff handle you are allowing.
One common mistake is to mark out a 4" splice but find that when the bow is narrowed to final width the splice has lost a lot of length.
Draw up the final width on the back of the billets and mark the splice to length on that and continue the lines out to full width. (Hope that makes sense)
Last one I did was a Z of about 3.75" length so that gives 11.25" of glue line
Judging by that I'd suggest going to 5" V or a Z.
Just checked my copy of "Billets to Bows" (a good read) he shows the 'Z' and 2 prong 'W' being 4" and the V being about half as long again)
One cast iron belt and braces trick which I used on a repair is a very shallow scoop off the back of the finished splice, overlapping the ends and a Yew sapwood patch over the splice and bulging slightly over the scoop (This makes a good looking slight swell at the grip).
I've used a similar trick when splicing Yew belly billets (for a crossbow prod) which will have a continuous backing, you can get away with a very short splice then
Del