How should I choose the resistors in a voltage divider?
The "resistors in the voltage divider" presumably refer to Resistor 6 and Resistor 7 in the schematic, providing the biased signal voltage to one of the inputs of the op-amp.

These dividers would be selected such that their combined resistance in parallel is significantly smaller than the input impedance of the op-amp. Otherwise, the current into the op-amp input will change the bias voltage - like putting an additional resistor of comparable value in parallel with one leg of the divider.
Also, they should be of high enough value that the current through them does not draw an unreasonable amount of current from DC Voltage Source 1.
Finally, the resistors should be of sufficient power rating to tolerate the power dissipation P = I^2 x R for the current drawn through them, I = V(DC1) / (R1 + R2)

The op-amp gain resistors would depend on the gain desired. Since the schematic does not indicate inverting and non-inverting inputs, it is not clear (at least to me) whether this is intended as an inverting or non-inverting amplifier, and what the intention of that part of the design is.
Dynamic voltage dividers can be implemented in a variety of ways, which include using a digital potentiometer IC, an resistor array with switched legs, etc. 

If one assumes that the question envisages only 3 possible input signal voltages, a dynamic voltage divider may not be needed. One could structure 3 voltage dividers, and switch the appropriate one in using a very small Rds(on) MOSFET, such that the MOSFET's resistance does not discernibly impact circuit behavior.
Just 2 voltage dividers would suffice, if the intention is merely to provide a counter-bias of -1 Volts for the 0-2 Volt signal when that is the source.

Overvoltage protection (presumably referring to the input signal voltage) can be achieved by using a voltage limiting zener diode circuit, at the simplest level. 

Depending on the currents involved and the behavior desired, other solutions such as voltage foldback or SCR-based crowbar circuits may be called for - apparently not applicable in this case.