Partly the size of the material, also seeing what shaped handle they think the material will do.
Also part of it is you learning the material do's and don'ts so that when you buy the material it has a better than average chance of shop approval.
It helps to learn if it's a left or right FWI tusk, plus learning about how deep the color is will influence the way you order the knife.
damn to wordy picture of FWI end inserted here...this piece made knife in next post, can go on if you are still interested just ask.
The ones above don't have super deep color like this piece in the picture, so best to not shape it & just a butt plate on the end.