Most of the AM medium wave transmitters for Part 15 low power do not use transformer modulation, with one exception being the Ramsey AM25 which has a small hand-wound transformer. But the shortwave transmitters on the blogs titled Talking Pixie2 and Big Talker got their start with a schematic that shows a small Radio Shack transformer, 8-ohms to 1K, with no stated power handling capacity. Even though that transformer does function, I checked around and found a Triad audio transformer with a rating of 1-Watt, which seemed more in line with the circuits being used. It remains to be tested.
Right now I am only interested in your point of view about low power, Part 15, modulation transformers simply as a topic of conversation.
In the adult-sized world of power radio, making a comparison, a plate modulation transformer for a 50kW station might way a ton or more.
Down here in micro-radio, what kind of power rating should a modulation transformer have?