No, I don't think the length has anything to do with it. If you think about it, as you increase the length of the axon, you increase the # of nodes of ranvier, which increases the # of Na+ channels.
the ways to increase increase velocity would be to:
increase axon diameter/radius. This causes less resistance to local currently flow and have faster impulse conduction
effect of myelination vs. no myelination
myelinated axons can conducts signals faster than non-myelinated axons.
You might be referring to length constant?
length constant=sqrt(membrane resistance/internal neuron resistance)
As you length constant increases, the magnitude decreases. (but not the speed)
ie. if you had a 1nm long axon, and let's say 50% of the signal passes to the end.
When you elongate the axons, to let's say 2nm, you might just have 25% of the signal pass to the end.
Lengthening the axon length has nothing to do with speed, but possibly with the voltage magnitude.