I've been rereading some of her earlier books recently and there was a description in one of those that described the general body type for a companion. It was the first book in the Wind series and from memory it was Karal describing them.
I'm sure there would be diversity as well but there would be a general body type that should work for most and you could dial in parts of other morphs to get something like Albrech's horse or the cob...
You're right Karal does wax eloquent on the beauty of Rubric's horse, and then later as he sees them in Companion's field. Though again, in the case of his admiration of Rubric's Companion, it could be how the one horse looks. Though I do admit, since they're all supposed to have insane stamina and speed there should be something to convey a horse with staying power.
You could always try writing Lackey, or her publisher to ask. You might get an answer.