Ship sizes

Well, there is an obvious size differential from Fighter -> Frigate -> Cruiser -> Capital Ship (and the flagship is a default capital). I am really personally hoping they add some mammoth type ship. Unfortunatly, there are none in Beta 4 and they have said that ships they made so far are final. So unless they are holding back from us or they are planning an expansion, there will be no colossal ships. The ships look the size they should though... don't know how else to answer your question.

ok thanks.

10m for fighters up to 1000m for the largest capitalship (obviously structures, asteroids, planets, and stars can be much larger). Every structure and ship has also had its size adjusted to the proper levels for final.

To make it short, what Blair wanted to tell you:
Youll see it

Yeah figured that out! ^^ Thanks

How is the modabiity of the game? Could Moders make bigger ships?

The TEC are getting another unit, the Novalith Cannon, on release. From the screenshots (and the name) it looks to fall firmly into the "mammoth" category.
One can only hope the other species get their own super-weapons as well.   

Balance dictates they will.

so do the devs.

High, very High.

It's easily possible to make bigger ships, if you want to.