How should a distribution transformer be sized for kVA?
Perform total load calculations per NEC, typically using Aricle 210 for branch circuits and 230 for services. Once the load is determined calculate kVA. If future loads are known adjust the number accordingly. Typically it is recommended that an additonal 25% load is added for potential future growth. Use this number and then size to the transformer that will cover this amount.
Note additional considerations may be made to use k-rated, low temperature rise or harmonic mitigating transformers.