How to get the scale bar of an FFT for a 2d image
If you are having trouble visualizing the reciprocal space, perhaps an analogy to 1D temporal FFTs will help (personally I'm more familar with these, so an analogy helps me anyway)

In a 1D temporal FFT, the input is N samples, at a rate of Fs samples/second, for a total time of N/Fs seconds.  The output is N samples, distributed between 0 and Fs (or between -Fs/2 and +Fs/2 depending on your point of view) (units 1/second).  So each output sample has width Fs/N (again units 1/second).  

In a 1D spatial FFT, all we do is replace seconds by meters and samples by pixels. The input is N pixels, and a rate of Fx pixels per meter, for a total width of N/Fx meters.  The output is N pixels, distributed between 0 and Fx (or between -Fx/2 to Fx/2) (units of 1 / meter).  So each pixel has width Fx/N (agains units 1/meter).

So an output scale bar of M pixels has a width of M*Fx/N in units of 1/meters.

To go from 1D spatial FFT to 2D spatial FFT, just need a sampling rate in each direction.  Presumably you have the same sample rate in each directin.