Estimating wattage of unknown autotransformer
This transformer is not designed to be an autotransformer. It is designed to provide 50V, 30V, 18V, or 19V from either a 110V or a 220V supply. If using a 110V supply then you would get 50, 30, 18, or 19V on their respective secondaries by connecting your supply between the 0V and 110V primary taps. If using a 220V supply then you would connect your supply between the 0V and 220V taps. If I am understanding you correctly you are ignoring the secondaries and are using all three primary taps in an autotransformer configuration. This is a very bad idea, as it removes all isolation from the mains, introducing a severe shock hazard. I am willing to bet this is not how it was done in the original design, and I cannot recommend you proceed. If you need a step-down transformer, get a dedicated one with a 220V primary and a 110V secondary. Please don't go the autotransformer route.

Anyway, to answer your question, a general rule of thumb is about 50 VA per kg. If you weigh the transformer you can multiply the mass by 50 to get a rough VA value. More precise numbers can be acquired by diving into transformer design math, but I doubt you'll want to go that far. Also, I cannot guarantee that this 50 VA/kg is accurate when you're trying to use a standard transformer as an autotransformer. That's just asking for trouble since you give up the transformer's isolation ability, so I will re-iterate, please get a dedicated 220V:110V transformer instead.