I am having some trouble with HOMER in that I am getting some illogical combinations (such as a 120kW PV array paired with 60kW of inverter, etc) in the optimised results. Homer sometimes seems unable to select the Conv-PV combo where it satisfies the condition that a Converter that is to be >= to the size of the PV. I realise that HOMER trys all combinations specified in the search space, but it doesn't seem to tag the previously mentioned combo as illogical. Is this on purpose? Is there any way to force the PV and Converter search spaces to be the following: Conv = PV or Conv >= PV?

It isn't necessarily illogical for the converter to have a smaller capacity than the PV array.  A smaller converter may mean some energy gets dumped, but that may be an acceptable trade-off for the reduced capital cost.  If the system includes a battery bank, the dumped energy will probably be very small.

But note that if HOMER reports that the optimal configuration has an undersized converter, you are welcome to choose another configuration, one with a converter equal in capacity to the PV array.  You should see all the sub-optimal configurations on the Optimization Results tab when you click Overall.