N step fft in D language

[Kadir Erdem Demir]

I am using fft function from std.numeric

Complex!double[] resultfft = fft(timeDomainAmplitudeVal);
The parameter timeDomainAmplitudeVal is audio amplitude data. 
Sample rate 44100 hz and there is 131072(2^16) samples

I am seeing that resultfft has the same size as 
timeDomainAmplitudeVal(131072) which does not fits my 
project(also makes no sense) . I need to be able to divide FFT to 
N equally spaced frequencies. And I need this N to be defined by 
me .

Is there anyway to implement this with std.numeric.fft or can you 
have any advices for fft library?

Ps: I will be glad to hear if some DSP libraries exist also