Tactical or Strategic?
Hex Grids are a must for board games, don't really need them in a computer game. In a computer game, the X,Y,Z coordinate system works best. I think, what you are asking is what kind of numbers do we want to deal with.
Meters are definately too small, Kilometers probably are too. I'm torn between hundred kilometers and thousand kilometers. At a thousand kilometers, that's a huge gap between two ships at range 1. Even at hundred kilometers its still a big gap, but it adds the next problem, Transporter range is several thousand kilometers. Do we really want people beaming 40 units away? I think thousand kilometers is best, but that's a lot of detail lost, and the ships would be huge compared to what they really are, otherwise you wouldn't be able to see them, even at range 1.