Wiring WS2812b to power supply
The longer the wire, the more resistance you have.  This is a known quantity that can easily be calculated by several online calculators.
For instance, using the linked calculator, and assuming 10 feet of 24 gauge wire (typical ethernet cable), resistance added by the wire is 0.257 ohms.  V=IR tells us that the voltage drop in the wire due to current is proportional to the resistance.  It seems that a single LED can draw up to 50mA.  V=0.05*0.257=0.0129V drop.
As you pull more current, that voltage drop number goes up.  So, at the end of 10 feet, if you started out with 5V, by the time you get to the LED, you would only have 4.9871V.  Not a big deal since the voltage on these LEDs needs to be above 3.3V.