Calculate required toroidal core size(Cross section area) for transformer
There's the hard way, and the approximate way.

The hard way is to pick a core size. Calculate how many turns, and so the length of wire. Calculate losses in the wire and the core. Then calculate the voltage regulation, and the efficiency versus throughput, and see if your application specification can live with those figures. Increase or decrease the size of the core for the next iteration if necessary. 

There is no 'right' figure for these, you may be happy with 10% regulation, you may need 5%. The efficiency tradeoffs are different for an standby power transformer that spends a long time at no load, and a microwave oven transformer, that only works at full load.

The approximate way is to make a list of toroidal transformers available on line from various retailers. Put weight and rated power into a spreadsheet and sort. Once you've found the consistent trend, take the weight of a transformer of your power, and halve it, assuming half the weight is iron, and half copper. Cores tend to come in a fixed aspect ratio, choose a size of core that is the weight you've calculated.