I can't tell you how long the ships are, but this may be of interest.
I just want to state that those numbers are there purely for flavor purposes and are not to be considered canon in any way, shape, or form
This is a prudent and wise stance.
But that won't stop me from rambling a little.
At 200 million tons, an Onslaught battleship, if constructed with steel in the manner of a current naval ship would be around two kilometers in lenghts.
Of course this is wildly inaccurate because we don't know what materials are supposed ot be used to build it and it is insanely armored which would make it far denser than a steel structure.
So its length based on this mass would be anywhere between a few hundred meters and a kilometer.