A1, if the transformer is delivering X volts, then the voltage of the diodes must be 1.4 times X volts or greater.
A2. Very difficult, the best way is to get a bit of low voltage AC, say 6V from a bell transformer and wire it to a low voltage winding (thick wires) and measure the input voltage from your bell transformer and the output voltage on the other windings. eg, voltage from bell transformer = 4V, voltage on windings, 17 V, and 80v. This would mean that the 80V winding was the mains (230V), the 17V would be 17/80 X 230 ~ 45V and the 4 V winding was actually 12V.
The actual power depends on how hot the transformer can get. Best to compare with a known example.
Frank