I think any degree of intermodulation generates an infinite series of harmonic combinations. There is no definite bandwidth but the power of individual spectrum lines increase.
As the spectrum bandwidth is usually defined to half-power or -3 dB, you can specify a reference level for it and then neglect all spectrum lines laying below that level.
I don't think that you need convolution to see why the bandwidth of intermodulation products is multiplied along with the center frequency of input bands. Just put band corner frequencies into the calculation.