Another thing to keep in mind about transformers is that they transform voltages and currents, but power (voltage multiplied by current) stays the same (less some relatively small losses; nothing is perfect).
As a simple example, if you have a step up transformer that has a 10:1 secondary to primary ratio, it will put out 10x the input voltage. Put 120 in, you'll get 1200 out. You'll also get 1/10 of the current that flows in the primary - if you pull 100mA (0.1A) from the secondary, there will be 1A flowing in the primary (ignoring losses). This would be considered to be a 120W transformer. Conversely, if the ratio is opposite with 1 turn on the secondary for every ten on the primary, the output voltage would be 1/10 that of the input (12V), but for that same 1A primary current, you would draw 10 amps. It would still be considered a 120W transformer, and would be about the same size as the previously mentioned step-up unit mentioned earlier.
Other than for transformers at the extremes (insanely high voltage or insanely low current, either of which might require specialized construction due to the need for insulation in the HV case, or very heavy wire in the LV, high current case), you'll find that transformers of a given power handling capacity are generally about the same physical size. Power capacity can be determined (if the secondary voltages and currents are listed) as someone mentioned above by multiplying each voltage by the associated current to get the power, then adding the power values to get the total. In short, just looking at a transformer usually won't tell you much about its input or output voltages, but will give you a feel for how much power it can pass.
I hope this makes sense rather than muddying the waters for you.
-Pat