fft and inverse fft error size and applying impulse response matlab
I don't know your overall goal but if you want to test the MSE associated with FFT / IFFT then you should perform the following

num_of_samples = 2819519 ;
fft_len         = 2^nextpow2(2819519) ;

X = fft(signal, fft_len);
y = real( ifft(X, fft_len) );
mse = sum( (signal - y(1:num_of_samples) ).^2 )/num_of_samples