user61818: Some sensors give as output a voltage between 0 and 5V. The analog inputs of the NodeMCU require voltages between 0 and 3.3V. I plan to make a voltage divider with two resistors like in this link for compatibility. 
To provide the desired voltage the quotient of the values of the resistances is fixed (approx. 1/2 for this case). However, the absolute values of the resistances are not.
Q: How to choose those absolute values? Which are the criteria and why?
I have a good background in electromagnetism but I know very few about electronic Engineering.
I think that low values must work, e.g. 1Ω and 2Ω (and surely greater values). But I am unsure of VERY high resistance values because I suspect that there should be some current intensity requirements. 
Thank you.

user57814: The low end of the values is limited by how much current your sensor can provide. If you use 1Ω and 2Ω resistors in series then your sensor will need to provide 1.67A to create 5V across the divider. I doubt that your sensor can do that. For most sensors, you would want to use resistors in the kilohm or tens of kilohm range.
The high end is limited by the input impedance of the analog input pin. If your resistor values are large compared to this input impedance then the input impedance of the analog pin will affect how the voltage divider works. If the analog input is multiplexed and the ADC has a sample-and-hold at the input, then very large resistances in the divider will not allow the sampling capacitor to charge fast enough. For most ADC inputs, you would want to use resistors in the tens or hundreds of kilohms.

user69172: To better understand voltage divider, one can look at this online calculator https://ohmslawcalculator.com/voltage-divider-calculator to see the actual formula and try the calculator to compute the unknown wanted.
It is not well publish the existing voltage divider on the NodeMCU, of course you need to browse the schematic to figure it out.
It goes the same with existing pullup resistor for some GPIO.
@Majenko your explanation is excellent!

user62523: As @Majenko commented, the A0 input has a fixed voltage divider (see NodeMCU Schematics) that connects the A0 input to a 220kOhm resistor, then to the ADC pin of the ESP8266, then to ground via a 100kOhm resistor. 

This effectively fixes the resistor values that can be used to scale the input of the NodeMCU. For a 0-5.0V input range (with sufficiently low impedance) you need to scale the ADC pin from 0-1.0V by adding a ~180kOhm resistor between the 5V output and the A0 input of the NodeMCU, creating a 400kOhm-> ADC -> 100kOhm divider like this:

simulate this circuit – Schematic created using CircuitLab

61818: <p>Some sensors give as output a voltage between 0 and 5V. The analog inputs of the NodeMCU require voltages between 0 and 3.3V. I plan to make a voltage divider with two resistors like in this <a href="https://learn.sparkfun.com/tutorials/voltage-dividers/all" rel="nofollow noreferrer">link</a> for compatibility. </p>

<p>To provide the desired voltage the quotient of the values of the resistances is fixed (approx. 1/2 for this case). However, the absolute values of the resistances are not.</p>

<p><strong>Q:</strong> How to choose those absolute values? Which are the criteria and why?</p>

<p>I have a good background in electromagnetism but I know very few about electronic Engineering.</p>

<p>I think that low values must work, e.g. 1Ω and 2Ω (and surely greater values). But I am unsure of VERY high resistance values because I suspect that there should be some current intensity requirements. </p>

<p>Thank you.</p>
 57814: <p>The low end of the values is limited by how much current your sensor can provide. If you use 1&Omega; and 2&Omega; resistors in series then your sensor will need to provide 1.67A to create 5V across the divider. I doubt that your sensor can do that. For most sensors, you would want to use resistors in the kilohm or tens of kilohm range.</p>

<p>The high end is limited by the input impedance of the analog input pin. If your resistor values are large compared to this input impedance then the input impedance of the analog pin will affect how the voltage divider works. If the analog input is multiplexed and the ADC has a sample-and-hold at the input, then very large resistances in the divider will not allow the sampling capacitor to charge fast enough. For most ADC inputs, you would want to use resistors in the tens or hundreds of kilohms.</p>
 69172: <p>To better understand voltage divider, one can look at this online calculator <a href="https://ohmslawcalculator.com/voltage-divider-calculator" rel="nofollow noreferrer">https://ohmslawcalculator.com/voltage-divider-calculator</a> to see the actual formula and try the calculator to compute the unknown wanted.</p>
<p>It is not well publish the existing voltage divider on the NodeMCU, of course you need to browse the schematic to figure it out.</p>
<p>It goes the same with existing pullup resistor for some GPIO.</p>
<p>@Majenko your explanation is excellent!</p>
 62523: <p>As @Majenko commented, the A0 input has a fixed voltage divider (see <a href="https://github.com/nodemcu/nodemcu-devkit-v1.0/blob/master/NODEMCU_DEVKIT_V1.0.PDF" rel="noreferrer">NodeMCU Schematics)</a> that connects the A0 input to a 220kOhm resistor, then to the ADC pin of the ESP8266, then to ground via a 100kOhm resistor. </p>

<p><a href="https://i.stack.imgur.com/QGkPV.jpg" rel="noreferrer"><img src="https://i.stack.imgur.com/QGkPV.jpg" alt="enter image description here"></a></p>

<p>This effectively fixes the resistor values that can be used to scale the input of the NodeMCU. For a 0-5.0V input range (with sufficiently low impedance) you need to scale the ADC pin from 0-1.0V by adding a ~180kOhm resistor between the 5V output and the A0 input of the NodeMCU, creating a 400kOhm-> ADC -> 100kOhm divider like this:</p>



<p><img src="https://i.stack.imgur.com/3rS2N.png" alt="schematic"></p>



<p><sup><a href="/plugins/schematics?image=http%3a%2f%2fi.stack.imgur.com%2f3rS2N.png">simulate this circuit</a> &ndash; Schematic created using <a href="https://www.circuitlab.com/" rel="noreferrer">CircuitLab</a></sup></p>
