No. The QT GUI Vector Sink does not have a fixed maximum vector size. Its Vector Size parameter simply has to match the length of the vectors presented at its input, and this size cannot be changed while the flowgraph is running.
Vector sizes of 1024, 2048, 4096, 8192, and even larger are commonly used in GNU Radio FFT applications.
If your flowgraph accepts 256 and 512 but rejects 1024 or 4096, the cause is almost certainly elsewhere, for example:
- The Stream to Vector block still has length 256 or 512.
- The FFT block size doesn’t match the vector length.
- A Complex to Mag², Log10, or another vector-processing block has a different vector length configured.
- A variable used for the FFT size is not updating all blocks consistently.