The general rule of thumb is that a transformer's kVA is the upper limit of the kW-AC rating of your solar system.
For example, if you have a 2000kVA transformer, you can handle a 2000kW-AC solar system. If you wanted to convert this to a kW-DC solar system rating, simply multiply your kW-AC rating by the inverter's AC:DC ratio, which is usually around 1.20. In the example above, a 2000kW-AC solar system would translate to a 2000kW-AC * 1.20 = 2400 kW-DC system. If your solar system is above your transformer's limit, you can either purchase a new transformer or simply downsize your solar system.