The NEC won't help you.
NEC 310.15(B)(6) will tell you that you need a certain minimum size of wire for a 100A service: 4ga copper or 2ga aluminium.
But the NEC won't tell you what to do in order to prevent voltage drop. Voltage drop is a design issue; you need to figure out what sort of voltage drop you can tolerate, what loads will be causing voltage drop, and what other system elements will be causing voltage drop.
1) What are your voltage drop constraints; do you have extremely sensitive loads that cannot tolerate much change in supply voltage, eg. esoteric electronics, normal loads that can tolerate some drop (eg. normal incandescent lamps), or do you have extremely tolerant loads (heaters, electronically regulated lamps, etc) A common rule of thumb is to have no more than a 5% voltage drop from supply to final load, including both the service conductors, any feeders, and the branch circuit conductors.
2) What are the loads; are they steady or highly variable. Are there any 'inrush' or 'starting surges'? If there are any large motors, eg for heat pumps, then they can draw quite a bit of current during starting, which will cause an extreme voltage drop for the few moments that the motor is starting. This sort of load might cause unacceptable lamp flicker on a long service.
3) The service conductors are not the only issue; you can have 'voltage drop' (actually impedance) at the transformer, such that the supply voltage changes considerably with load current.
Chapter 9 table 8 will help you with conductor properties, which you can use to calculate voltage drop. There are also voltage drop calculators around the web.
-Jon
