How does amplitude scale when FFT size is smaller then signal duration?
When you perform simple DFT of your signal with length $N$, then you have to scale it properly. Normally it is done by dividing magnitude by length of spectrum vector, which is $N$. Another thing is that normally we are interested in one-sided spectrum, therefore you need to multiply your above result by 2 to compensate energy. This will assure, that values of magnitude are representative. 

In case when you are using windows different from rectangular (Hamming, Blackman, etc.) you are not dividing by $N$ - you must scale it by sum of your window samples.

Therefore when you are windowing your signal, you get a new signal with a length of a window. Thus in your case amplitude will be scaled down by 12.

Example for 1Hz sine wave and two window types is shown below. Please note, that frequency axis is chopped at 3Hz. You can see that amplitude of peaks is equal to unit amplitude of signal. Also in case of Hamming window side-lobes are much lower than in case of rectangular, sacrificing main-lobe width at the same time. This can be seen better in logarithmic scale.



MATLAB code used to produce result:

clc, clear all, close all
f0 = 1;        % fundamental frequency
fs = 50;       % sampling frequency
T = 6*pi;      % upper time limit 
t = 0:1/fs:T;  % time vector
N = length(t); % number of samples

% 5 Hz sinusoid with unit amplitude
s = sin(2*pi*f0*t);
% Windowing signal
win = hamming(N)'; 
s_win = s.*win;
% Magnitude spectra
S = abs(fft(s));
S_win = abs(fft(s_win));
% Frequency vector
f = (0:fs/N:round(fs/2));
% Take one-sided spectrum
S_h = 2*S(1:length(f));
S_win_h = 2*S_win(1:length(f));
% Normalisation
S_h = S_h / length(s);
S_win_h = S_win_h / sum(win);
% Time domain signal plots
subplot(2,1,1)
plot(t,s)
xlim([0 T]); grid on; hold on;
plot(t,s_win,'r');
legend({'Rectangular window','Hamming window'})
xlabel('Time'); ylabel('Amplitude')
% Frequency domain plots
subplot(2,1,2)
plot(f, S_h)
grid on; hold on;
plot(f, S_win_h, 'r');
legend({'Rectangular window','Hamming window'})
xlabel('Frequency'); ylabel('Amplitude')
xlim([0 3])