Answer:
To size a 24 volt transformer you have to work backwards from the load amperage. Small transformers are rated in VA(volt amps), for example if a doorbell chime draws 1.5 amps then the transformer needed will be 24 x 1.5 = 36 VA transformer. Since these transformers only come in specific sizes all was select the next biggest size up from what you need. In this case a 50 VA would be needed. The transformer rating usually advance by 25 VA at a time and then by 50 VA when the sizes get larger.