psd analysis by calculating powers and peak frequences

Can anyone tell me how I can analyze the PSD by calculating powers and peak frequencies for different frequency bands? These are my frequency bands: ULF <0.0033 Hz VLF, 0.0033-0.04 Hz, LF 0.04-0.15 Hz HF 0.15-0.4 Hz.

This is code I'm using for the PSD calculation:

fs = 400;
xdft = fft(odstepRR);
xdft = xdft(1:length(odstepRR)/2+1);
xdft(2:end-1) = 2*xdft(2:end-1);
psdest = 1/(length(odstepRR)*fs)*abs(xdft).^2;
freq = 0:fs/length(odstepRR):fs/2;
plot(freq,10*log10(psdest));

[Wayne King]

what is the length of odstepRR?