I think we just had this discussion recently. The Sivar's weapon can not target ships, but in terms of the sheer amount of damage it causes it is significantly more powerful. The Concordia's weapon can destroy "any ship", but does less damage than the planet-destroying gun.
Re: Sivar's size. It's hard to say... it's described as being large in the game, but the most 'official' Claw Marks specifications (from Super Wing Commander) actually say it's tiny - 103 meters.
The CGW insert done at the time says 550 meters - which is large but not *huge*...
... and then the movie features 825 meter Sivar dreadnaughts that seem to be a ship with a different role.