That is very little power for its size, perhaps it is a control tranformer. To answer your question: I=E/R = 15.7/81 = 194mA short circuit output. Maximum power will be with 81 ohm load: I=97mA. The primary current can be calculated from the turns ratio: 230/15.7=14.6, 97/14.6 = 6.6ma. 6.6mA*653 ohms = 4.3 volts drop in input voltage; not a lot.
From the physical size it sounds about 2 to 3VA, which means at best 50 to 100mA max on both secondaries, the 81R is a high, so the regulation will be poor.
Connect the mains and load with a 220R 3Watt resistor, check the 17.5Vac
If the 17.5 volts is the open circuit voltage, the output to the 81 ohm load will be 17.5/2=8.75 volts. If you connect the two secondaries in series, you can get 17.5 volts to a 162 ohm load.
God I feel like a complete idiot right now...
Could you please tell me how many mA will I get when I regulate the voltage to 12V, 5V or 3.3V. One secondary only (I use 2 windings separately to get 2 voltages out of it in one go)