In reply to Neily:
Hi Neily,
Good Question. Yes, it would seem a logical assumption that a shorter shaft length will slightly reduce the holding power of a buried axe belay (although I don't know of any data to support/refute this view) simply because there is less axe to resist shearing through the snow.
However, the effectiveness of an axe belay relies on several other factors such as the type of snow, careful construction of the belay and the belaying technique of the climber - I think that a slight reduction in axe shaft length will be far less of an issue than these other factors.
Thanks for raising the point though.
Cheers,
Paul