Samples per second in a row vector in Matlab?
Duration is just the length of the signal. half makes the duration half, and double makes it double!
Pitch is the auditory sensation of the frequency. If you double the length of a signal (given the rate of playback remains the rate of sampling) then it will sound like a lower frequency. Similarly, when you make it half, it will sound like a higher frequency.


The code you can use to hear it:

fs = 8000;  % sampling frequency
duration = 1; % seconds
f0=100; % frequency
t=0:1/fs:duration; %time axis
wave = 10*sin(2*pi*f0*t);
sound(wave,fs) % original tone
pause
half_wave = wave(1:2:end);
sound(half_wave,fs) % half duraton, double pitch
pause
double_wave = interp1(linspace(0,1,numel(wave)),wave ,linspace(0,1,numel(wave)*2-1));
sound(double_wave,fs) % double duration, half pitch