If you are working in 3D Studio Max, it looks like the standard block is 100x100x100 units with the origin on the bottom of the object. I've used this to get started making prefabs, but if I'm wrong, at least it is a solid number and should be easy to scale to fit.
I see the format-of-choice is OBJ. I'm really hoping support for something like 3DS might be in the future, as at least 3DS has primitive animation support. With the ability to link objects to one another and animate them, prefabs become a lot more interesting. Imagine an engine with moving parts inside, fans in ventilation ducts, or even fake-holograms using moving 3D parts.
Maybe I'm getting way ahead of things, but since I can't put anything in-game yet, I'd like to be able to get started making the assets in such a way that they'll be ready to go when the time comes. Not only am I a big fan of this game, but now that everything in the game-industry has to go through a mudbox/zbrush ultra-high-res pass with all kinds of tricks and shortcuts to get more and more detail, I'm looking forward to some good old fashion low-poly modeling.
A few things I'm working on: A switchboard for your cockpits (Align to 45 degree blocks some day?)
An access hatch that only occupies one block, but will require animation to be useful.