Found the quotes I was looking for, which really show a larger ship than DDs is better. Maybe not so high as BC, but a CA is definitely a winner.
Quotes from Honor Among Enemies, Chapter 15
The problem lay in the new data Commander Hauser had provided. Raiding patterns had shifted since ONI put together her own pre-deployment background brief. Ships had been disappearing in ones and twos in Breslau and the neighboring Posnan Sector, and they still were. But where whoever it was had been snapping up single ships and then pulling out, so that the next half-dozen or so got through safely, now as many as three or even four ships in a row were disappearing—all in the same system. Losses were actually higher now in Posnan than in Breslau, which was what had forced Honor to rethink her original deployment plans, but the new pattern of consecutive losses was almost more worrisome than the total numbers. Consecutive losses meant raiders were hanging around to snatch up more targets, and that was wrong. Raiders shouldn't do that . . . or not, at least, if they were operating in the normal singletons.
No raider captain wanted to stooge around with a prize in tow, because two pirates carried sufficient crew to man more than two or three—at most four—prize ships unless they captured the original ships' companies and made them operate their ships' systems.
~ ~ ~ SNIP ~ ~ ~
"You know what I wish?" Truman asked. Honor looked at her, and the other captain shrugged. "I wish we knew who was funding and supporting the bastards. You know as well as I do that the average piracy ring can afford to lose and replace vessels—and crews—all year long if as much as a it. These eleven ships"—she tapped her screen, where the names of the most recently missing vessels were displayed—"represent an aggregate value of almost twelve billion just for the hulls. You can buy a lot of ships heavy enough to kill merchies for that kind of money."
No pirate ever truly operates "solo" such as we're really debating. Pirates always have a logistical tail: their fences, personnel managers to get new crews, corrupt officials, and so on. And so long as you perform, meaning taking more than a few prizes before you get caught, and lose your ship, your backers will supply you with a new one so you can get back to work making them money.
And now we've got a rough pricing of ships of the time, circa 1908. 11 freighters lost, worth twelve billion just for the hulls, and that you can "buy a lot of ships heavy enough to kill merchies", which could be anything BC or lower. That's some of the best actual market data I think is in the books at all, when you exclude anything that includes wallers.
And it also gives some modus operandi, about pirates. Normally grab one prize, and then run with it, and then they changed to snap up multiple. Now the multiple snaps was probably the Peep scouting forces, or it might have been Warnecke's lunatics. But it appears to be
just common enough that nobody at ONI even considered it to be Peep commerce raiders. To snap up multiple targets in one system, requires the larger hull. Which we've just seen is affordable even to run of the mill pirates, but also generally takes two heavy cruisers (the Peep raiders that tried to nail HMAMC Scheherazade at Tyler's Star)