As a first estimation, you can give it double the number of turns as needed for a mains transformer of respective voltage , resulting in a full power lower bandwidth corner of 25 or 30 Hz, which should be o.K. for low quality public adress as usually intended with 100 V systems. So the core size is equivalent to a 500 VA 50 Hz transformer.
If upper cut-off frequency of the fransformer above 10 kHz is an objective, you would need to build the transformer different from a standard mains transformer with interleaved sections to reduce the leakage inductance. As far I'm aware of, it's unusual for industry standard 100 V transformers, but it's possible.
Finally, you should check, if your solid state amplifier is capable of driving inductive loads without getting damaged.
P.S.: I noticed, that some 100 V transformers available from catalog distributors are using considerable smaller core sizes related to nominal power. Apparently they are designed for a more restricted frequency range, e.g. > 100 Hz. It's not the quality that has been usual in German 100 V systems of the past.