Hi Nick,
It is necessary to define a criterium, and it is dependent of the application.
For example, the bandwidth of a filter or a spectrum is usually defined at -3 dB points, but other levels are possible.
In your case, is would be possible to define the bandwidth at -10 dB return loss (this corresponds to VSWR<1.92:1) or at VSWR<2:1 (correponding to -9.6 dB return loss). This would give a bandwidth from 5 to 6.5 GHz approx.
If you would adopt instead the criterium of WSWR<1.5:1 (another very usual, corresponding to return loss of -14 dB) the bandwidth would be narrow in this case. Nevertheless, the design could be tuned (approaching the two resonances so that te peak at 5.85 GHz decreases) in order to wide it.
Regards
Z