Either I understood that wrong, or it is not accurate:
The size is 2^order (or 1 << order), so size of 10 should be 1024.
The array for processing needs to be 2 * fft.getSize() = 2048, see the docs for FFT::perform().
The size of the array passed in must be 2 * getSize(), and the first half should contain your raw input sample data.
And the number of samples to supply are fft.getSize(), at the beginning of the array.