Billets can be pretty short.
I would keep 30 to 40 inch pieces of pretty wood for billets.
Short Bow and Kids Bow length from about 39 to 50 inches.
Pretty much the sky is the limit.
The folks on this site and others have built some Amazing short bows.
And Bows from Billets can be just as good as bows from uncut staves.
You can also do interesting layouts with billets that are difficult to do with straight full length staves.
If you haven't picked up a copy yet I would recommend the Traditional Bowyer's Bible set.
Four volumes, they can be had individually or all together.
-gus