TNG seemed to follow the rule that the enemy ship always had to be as large as the Enterprise
Umm, where?
Most of the adversaries were depicted as puny, with the big saucer of the hero ship hulking over them (starting with Ornarans, moving on to Okona and his troublesome almost-in-laws, the Klingon BoP, Pakleds, Acamarians...). Others were depicted the opposite way, their mighty shadow falling on the heroes (starting with the Romulans, then with the Borg, the Tamarians, the various space beasts). Basically only the Ferengi were given ships of "heroic size"; Klingons later got their corresponding design, but they always also had diversity.
Timo Saloniemi