[Rollo]

Thx Rollo. Is there any way though to have 'attack ships' that are above 1499 kt though? I've got the designcreations txt file set for 5000 kt attack ships but the only ones that get built are the attack bases. The ships seem to stop at the lowest kt below baseship size (my 1200 kt superdreadnoughts) and anything bigger must be an attack base, this even happens with AI's that don't even have attack bases in their txt files. Is this hardcoded?

yes, it is possible and I have a hunch why it isn't working for you.
What is the 'Minimum Speed :=' for the big attack ships in the design file?
And what is the 'Requirement Max Engines :=' for the Baseship hulls?

If the minimum speed is greater than the number of allowed engines, the hull cannot be selected by the AI, since it doesn't fullfil the minimum speed requirement.
Lower the 'Minimum Speed :=' for designs 1500kT - 5000kT and it should work.