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.
Some sources
Probes | Henry's Avalanche Talk
Avalanche Probes - Snowshepherd
http://www.avalanche.ca/adx/aspx/adx...obe+review.pdf
I'd be interested to here some more perspectives on this before proceeding.