Thanks for your reply Kari.
Well my question was "double". I wanted to know what is the smallest resistance I can put to have maximum precision on my measurement and minimum risk on my Arduino board.
I'm just going to connect VCC to pin 3 (for example) with the resistance I want to measure. Then pin 3 to the resistance of 100 Ohms which will go to the ground.
VCC pin 3 (analog read)
unknown R1 | 100Ohm R2
If I understand well, if my unknown resistance is zero, then total resistance will be 100 Ohms and delivered current will be 50 mA (100 * 50 = 5V) which is within the specs. And I should not burn my board. The reading on my analog input should be very-very close to 5V.
If my unknown resistance is very big, amperage should be very low (good for me !) and the analog input should be very-very close to zero volts.
After this I just need to perform a calibration to know my range of readings and I'll be fine.
To tell you more what I want to measure: a soil moisture DYI sensor. Basically 2 conductor (piece of metal or pencil carbon thingy) in a shaped plaster of Paris stick. I'll stick this in my plant ground and log the values of the resistance over time. Then I'll plot this on excel and see where I want to put the limits for a potential automatic watering system. (as you will have understood, I have no idea of what kind of resistance I expect). http://www.cheapvegetablegardener.com/2009/03/how-to-make-cheap-soil-moisture-sensor.html
BUT, I have other ideas with photosensor, potentiometer, etc.One more question:
putting 100 Ohm resistor is great for measuring a value that will go from zero to an unknown maximum value ... how do I optimize my system for a resistor that will oscillate between 10kOhms and 12kOhms (or 20kOhms and 30kOhms ... or what ever other value !) ?