I'm sure it can't hurt too much to have a 3m+ one, though pretty much all of the info I've read so far (in the links stuck above) and on European based sites suggest 2.40m is the ideal length a personal pack. Covering typical burial depths comfortably while being easier to deploy and handle. Longer, heavier duty being best for more frequent use applications such as organised search teams. To me this does seem like intuitive advice.
Probes | Henry's Avalanche Talk
Avalanche Probes - Snowshepherd
I'd be interested to here some more perspectives on this before proceeding.