What is the smallest duration that can be measured using micros()?
What is the least value of time that I could measure(count) using micro() in Arduino board?


The answer is 64 clock cycles or 4 us @ 16 MHz. Please see the Arduino AVR core[1] for more details. 

Cheers!

Ref. [1] https://github.com/arduino/ArduinoCore-avr/blob/master/cores/arduino/wiring.c#L25