The relation the gives the dependence of the intensity of magnetic induction `B` on a solenoid axis as a function of its length `l` , its number of wire turns `N` and the current through it `I` is:
where `mu` is the magnetic permeability of the solenoid medium.
Therefore one can say that the longer the solenoid is the smaller the magnetic field produced by it will be for the same electric current through. Thus the length of solenoid is inversely proportional to the magnetic field it produces.
dud as i know the strength of solinoid is directly proportional to the no. of coils but i do not understand ur question, as it is about length of the coil so i cant really say anything....