They are application specific so it really depends what is it that you are protecting and from what you are protecting it. Obviously a consumer device you use indoors requires less protection than some equipment outdoors that must survive lightning strikes.
Even in your case, the "Ethernet" or "USB" really means nothing, as there are so many types of Ethernet (speeds, PoE or not) and USB. Each of them requires different kind of protection because they are different and have different requirements for protection.
There might be standards how much protection is needed, and you may want more protection in harsh environment.
Voltage must be high enough and capacitance must be low enough so protection does not interfere much with normal operation.
Voltage must also be low enough that the protected item can handle it.
Current depends on how much protection you need.
Selection of protection device depends also on how fast, or how long the surges are.
It also depends how much protection the protected item has built into it.
So rule of thumb is, read application notes about it, preferably ones that match your chip or interface type. Protection devices usually have a list of what they are suitable for.