To accurately measure various transformers we use a sweep generator, a known resistance, and an oscilloscope. In this image there are two ribbon mic transformers being measured here. The one on the little PC board is a Lundahl and the other is a Sowter. We have them all here, plus a bunch you've never heard of. An interesting one is the Oktava.
The are all quite different.
What are we looking for when we spec out a ribbon mic transformer? Primarily, it is efficiency. That's why larger conductors, and transformers, perform better in most of the required categories. Smaller transformers may have the appropriate ratio but then suffer high internal resistance, and there are some puny transformers of the past (like the B&O) that were so miniscule they hardly did the job. Certain considerations, such as hum aperture, internal shielding, and Ohmic resistance, compete with each other to some extent, and knowing how to manage that is part of an integrated total product design.
In guitar amps, microphone transformers, preamps and a lot of other things we find that high efficiency and low noise are the two most important attributes that contribute to a clean sound.