for a mos device there are parameters specified like Vto, µn , λ etc. i would like to know the value of the length (l) for which the λ value is given .
Do you mean the parameters given in a SPICE file? If yes then which level. The higher level models do not have λ as a parameter. Its always better to derive these parameters by plotting the DC curves of a MOS device of known length and Width for your hand calculations.