Now that HOMER Pro can model dedicated inverters for PV arrays, I am wondering what number to enter in the search space window. Should I use the DC kWp value of the array?
First of all, of course, you can check the box "Match parameters to PV". In the future this will be renamed "Ignore dedicated converter". You can still assign the PV to the AC bus, but the inverter capacity and efficiency will not be considered in the model.
If you do want to consider the converter in your analysis, then sizing to the PV array DC kWp output is a good place to start. The search space entry for the converter corresponds to the peak output power, so you may want to consider the converter efficiency, especially with lower converter efficiencies. For example, if the converter is 75% efficient and the PV array capacity is 4 kW, you could set the converter capacity to 3 kW and theoretically handle all the PV output.
I say "theoretically", because the PV output can exceed the rated capacity you specify in the search space, in some circumstances. The PV rated capacity is based on STC (1 kW/m^2 irradiance), and actual irradiance does sometimes exceed that -- this will depend on your solar resource. See the HOMER help article "How HOMER Calculates the PV Array Power Output" for more details on this.
Our intention with the separate search space for the dedicated converter is to allow you to optimize the converter size. If you input cost information for the converter, and select a range of sizes in the search space, HOMER will find the economic optimum. This can often result in an optimal converter size that is significantly smaller than the PV array capacity, since most of the energy produced can still be captured with a smaller converter. If you are planning on using an undersized inverter in your system, you'll have to be sure that it is capable of curtailing the solar output -- this is outside the scope of a HOMER analysis.