TNG seemed to follow the rule that the enemy ship always had to be as large as the Enterprise
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.
When they were dealing with the "Big Adversaries" they tended to inflate the size of the ships, at least from my view.
When they deal with somewhat harmless aliens, the ships were 'regular' size.
Look at the Ferengi ship in The Last Out Post. Then Cardassian Ship in The Wounded. Or the Tamarian ship in Darmok. Big suckers, pretty much the size of the Enterprise.
As far as TNG, it looks like they only used large scale ships for the Bird of Preys; they always looked roughly the same size as the Enterprise to me.
I think they went with capital ships because back then, because the battles were slower, shorter, and they wanted to establish a threat to the Enterprise.
Flash forward to DS9, and you never see the huge B.O.Ps again. They're all smaller, faster, yet still deadly.
The Jem Hadar ships were smaller but but had a huge advantage, which DS9 portrayed perfectly.