Switchable ferroelectric Langmuir-Blodgett films prepared from the ferroelectric copolymer of vinylidene fluoride with trifluoroethylene P(VDF-TrFE) manifest the true bistability for films thicker than 15 monolayers; thinner films are still switchable but monostable. The crossover of the switching behaviour presents the first estimation of the ferroelectric correlation length, xi approximate to 75 Angstrom.