There is no actual definition as to what exactly a \"Capital ship\" is. Just because you classify capitals as needing to be 400 meters long doesn\'t mean everybody does.
\"There is usually no formal criterion for the classification, but it is a useful concept in naval strategy; for example, it permits comparisons between relative naval strengths in a theatre of operations without the need for considering specific details of tonnage or gun diameters.\" http://en.wikipedia.org/wiki/Capital_ship
The first Dreadnought, The HMS Dreadnought, was considered a capital and In fact a was only 160m.
Also people are way to obsessed with gigantism in this game, probably thanks to movies/shows like Star Wars where their ships are impractically huge. The largest military vessel in the REAL world is a Nimitz Class Super Carrier and it is only 332.8 meters long. I honestly don\'t think we will be flying around in kilometer+ sized ships for real, ever.