@Roboboy: There will probably be ways to modify the shields, sure, but the devs have yet to let loose any such information regarding how they plan to do the shields. All they've said is that they're going to use Liquid Cubed algorithm; which leads me to believe that after you place shield emitters on your hull, that the sheild will act like a liquid, coating the blocks around it. More emitters, the faster overall coverage will be and the thicker the shields will become, able to take more damage at the expense of requiring much more power. That way, you'll be able to physically tell the damage you've done to the sheilds, as the coating on the hull blocks start to dim and crack and finally disapear completely under your attacks.
@HLBeta: One of the things made clear early on is that space is dangerous, space is deadly, and if you run out of air, you die. The devs know this, and we all know it. Systems like bulkheads, air purifiers, system-shutoffs and emergency repair systems could possibly be all included to help keep the players alive once they've messed up or gotten beaten half to death by someones plasma cannons. When and how these things actually go in is still up to the devs.
At the moment there aren't any restrictions on size, although the game engine (as of W16) has problems loading massive entities longer than about 600 blocks (600 meters), found thanks to Yolan. The W19 code, on the other hand, Gabriel was kind enough to test when I asked him, and said the range had increased to about 1200 meters in length, 300 meters in depth, and 300 meters in height before the system couldn't load the entities again. Mind, there will be personal limits do to how much RAM your computer has.
Also, the Warp Cores will be one of the few artificle limits on ship size. A single core won't be able to transport a 'Planet', as it were. As for using multiple cores... we'll just have to see how it all falls into play, yes?
Edit: Okay, I was mistaken. Gabriel was just kind enough to inform me that what you can save/load should be nearly infinite now, while what you can actually build is limited by your RAM. Gabriel's computer konked out at 1600 meters long, 600 meters wide, and 300 meters deep.