We do not have an exact value for the maximum signal length for CX3 SPI interface. This is because it depends on many factors such as cable,driver circuit, receiver circuit, frequency,signal levels etc.
For increasing the signal quality, please try the following suggestions:
1.Decrease the frequency.
2.Do length matching between clock and data lines.
3.Do impedance Matching.
Also, make sure that the SPI Timing Parameters that are mentioned in page 42 of FX3 datasheet is met properly. The link to FX3 datasheet is given below: