Hi,
Another way is to look at the wire thickness that makes up the windings.
You can then compare the wire gauge/thickness to a decent size for
transformer windings, like 600 circular mils per amp, which will give you
the amps. If you look at the primary and it comes out to 10 amps and
the primary voltage is 120, then you know it is a 1.2kW transformer.
Alternately, stand the transformer upright with side facing camera,
with ruler on the table right up against the side at the bottom.
That will give us the stack height nearly exactly.
Then, stand the transformer on its back (on one of the windings) so
that we can see the inside part of the transformer which is the center
part of the stack, with a ruler sitting on top so that we can see the
edges of the center stack AND the ruler. That will give us the stack
width.
This will enable us to give a better estimate too.