biochemistry - What limits the maximum spacing of Nodes of Ranvier and which organisms tend to have the widest gaps?

This is best answered via something called cable theory. Basically, as the action potential (AP) propagates along the membrane of the axon, it's tripping voltage gated channels that "renew" the flux of ions into the stream at the nodes. There are no ion channels under the sheathing (or they are there anatomically and inactive, but I don't remember), so the current is able to zip through that area. There's no renewal of AP, but there's no loss due to leak of ions either. Since there's no influx of ions from outside of the membrane until the next node, the capacitance and resistance of the membrane is exponentially decaying the voltage. So, if the distance between the nodes was too long, the voltage would just die off and the AP wouldn't propagate.

I don't have any anatomical data about individual species, but know that only the smaller diameter axons require sheathing to compensate for the slower ion flow due to higher resistance (e.g., the squid giant axon(not to be confused with the "giant squid" axon) has no myelination due to its large diameter).