Where were you when I was debating this in a World War I forum?
Depends on where said WWI forum is and at what time you were debating it. 
off-topic but as we are not going to get data from IC on titans soon...
Well, IRL, the size and armor would not matter at as it doesn't exist any armor that would protect against a direct hit (or even a proximity explosion) from a thermonuclear missile. Far more efficient than any laser, mass driver, phaser, whatever other weapon exist in Sci-fi. I, mean, the plasma wave will slice your battleship in two.
In space, the only thing that matter is the acceleration (to outrun or catch an enemy ship) and detection devices (if you spot your enemy without him detecting you, you can shoot your missiles without any exposure to retribution).
By the way, you are lucky if you have a radar that can detect anything beyond 100 000 km (and i am being very very optimistic here) and considering the size of the solar system, two enemy ships will have trouble to find each other.
If you want to have an idea of how space combat looks like, think today submarine battle, the kind of everything or nothing bloody brawl.
And much more knowledgeable people than both me and you put together disagree with your naive approach. We aren't going to rely on RADAR for detection.
We're going to use telescopes and thermographic imaging. Drives are going to be bright. The current NASA Space Shuttle's main engines can be seen from the asteroid belt, IIRC. So it's not going to be a case of "where's the enemy".
Further, a nuclear weapon is expensive, and, more importantly, very heavy. Spaceships are one of those things that simply cannot afford extra or useless mass, otherwise it impedes performance. A laser would be much better. Not only is the laser a better generalist weapon, it can be used as a PD weapon system to zap incoming missiles.
And for a nuke, it doesn't take much to make it not-dangerous.
Also, "plasma wave"? What are you talking about here? There isn't any "plasma wave" from a nuclear detonation in space. A nuke going off in vacuum is like a flash-bulb; a very, very, intensely bright flash, that lasts less time than it takes for your brain to tell your eyelid-muscles to blink. That's not to say a nuclear weapon wouldn't be dangerous, though. Any nuclear weapon that detonates within 500 meters of your ship is very probably going to slag your ship, and you.
But that's the problem, see. It has to get there to begin with. If it can't, then it's a waste of money. And if I just zapped your missile with my laser, and I can keep on doing so until you run out of missiles, then once you do run out of missiles, I can then zap you.
So really, generally-speaking, everyone sees everyone, and lasers are more likely to dominate than missiles.
That's not to say missiles are useless, per se, just that you need a lot of them, preferably very inexpensive, to overwhelm a laser-based PD system. At which point the lasership is pretty much SOL. But you have to overwhelm the laser, and that can take a lot of cheap missiles, or a few very expensive missiles.
That's one of the reasons why, IMO, nuclear weapons aren't likely to have particularly widespread usage in space warfare. They're expensive, and they're easy to neutralize.
EDIT: thought I should also address this:
There isn't much of a way to make a distinction between a battlecruiser and a pocket battleship, because pocket battleships can be classified as battlecruisers. I am well aware of the Deutchland-class pocket battleship, which can't even be compared to a battleship, and is like the HMS Hood being called a battleship by the British.
Well, in a wet navy context yeah, a pocket battleship and a battlecruiser are pretty much the same thing.
But in a space context they're very different. A pocket battleship is simply a smaller and cheaper battleship, but still a battleship. It'd be something a nation-state would build if either they needed to police a lot more territory than they could afford battleships, or if they can't afford battleships in the first place. A full battleship would likely be able to defeat a pocket battleship, but then again, a pocket battleship is simply a smaller and cheaper battleship, so it's presumably armed with weapons of the same firepower that the battleship carries. Likely, it simply carries fewer battleship-sized weapons.
A battlecruiser in a space context is fairly nonsensical. It's supposed to be faster, and less-armored, but of the same armament as a battleship. "Speed" isn't a focus in space combat. So that's strike one. If it has less armor, it's not as tough. Ergo, it's a lot more likely to get blown up. Strike two. It packs the same (likely expensive) armament as a battleship, but can't take the punishment a battleship could. Strike three.
A battlecruiser is likely to be too expensive and fragile for it to see any kind of service in a military setting.
Of course, that's if you use the fairly traditional definition of a "battlecruiser". If you instead define it as a fast, better-armed and armored cruiser, then you have something more useful. If you instead say, "A battlecruiser is a cruiser with battleship-ranged weaponry", then it could work. The former would likely find usage in, say, a merchant navy. The latter, well, I'm not really sure what use would be found for something of that type.