How can I specify the FFT length for the spectrum objects in the Signal Processing Toolbox 6.2 (R14)?

% Sample test data to perform the Welch method on:

t=0:0.001:2*pi;

x=sin(t);

% Initialize the Spectrum and Options objects

Hs=spectrum.welch;

Hopts=dspopts.spectrum;

% Change the options to use user-defined FFT length and Sampling Frequency

set(Hs,'FFTLength','UserDefined');

set(Hopts,'NFFT',512);

set(Hopts,'Fs',1000);

% Display the results

psd(Hs,x,Hopts)