Does anyone know where I can find a computer program for calculating the size of a solar PV system?

I would recommend PV-Syst (this is a pretty advanced tool and post solar engineers use this software.

Another good one is NREL's System Advisor Model (SAM). very easy to use.

Outback also makes a "String Sizer Tool" designed around their latest product lines . . .

There are actually some links you could follow on this article "String Sizing Tools"