Transformer selection about arbitrary signal generator
If you have 20 volts RMS and a peak current capability of 20 mA from your amplifier then that limits the power into a transformer to about 280 mW. Because a transformer doesn't amplify power, that means, after losses are taken into account, the max power from the transformer may be about 250 mW.

You require an output power of 1.414 watts so you need to insert an extra amplifier between your OPA   and the transformer to boost current and provide the required power.