In sizing a transformer used as secondary output for a generator, the following things have to be considered:
1. the size, in KVA, must not be less than the max KVA rating of the generator. remember that some generators have a service factor of 10% above the max nameplate rating.
2. the voltage and frequency also play a vital role in the selection of a transformer.
3. we do not size the transformers at 100% loading but we have to consider demand factor, usually at 85%.
4. i presume that that the size of the generator was computed based on the known loads (and future loads).