[Ragnarok-X]

Here are my suggestion, from own experience: Is the colonyship hull really 500 kT big ? If it is, can a rock colony component be packed on it, size-wise ? If so, then im almost 100% sure its a problem with your racial traits and empire setup.

If you are using the system which is used within the STM Mod, then you are most probably having 9 unique colonization tech areas (3 for each native). Be sure you have assigned the right colonymodules to the right tech families and the right racial traits for them in the empire setup. If you didnt, then the AI is not able of designing a colship at all, since the component is not available. My suggestion to test this is, make a new game, run turn one, switch over to the AI empires and design a ship, that way you will see if the AI has access to the RIGHT module at all. I know its confusing, but when using this quite unique system of colonization traits/technologies, things can easily get weird and confusing. Hopefully you understood my at all.