No. The Airspy Mini itself does not limit the FFT size to 256 or 512 bins. It simply streams IQ samples at supported sample rates (3, 6 or 10 MS/s), and GNU Radio can in principle perform FFTs of 1024, 2048, 4096 or larger.
If 256 and 512 work but 1024 and 4096 are rejected, the problem is more likely to be one of these:
- The Stream to Vector block length doesn’t match the FFT size. If the FFT is 1024, the Stream to Vector length must also be 1024.
- The window length must match the FFT size.
- A downstream block (such as a QT GUI Frequency Sink or Log Power FFT) is still configured for a different vector length.
- A variable controlling the FFT size is constrained or is not being propagated correctly through the flowgraph.