This is the entry I've been playing with mostly.
Code:
Name := Colony (Rock)
Design Type := Colony (Rock)
Vehicle Type := Ship
Default Strategy := Don't Get Hurt
Size Minimum Tonnage := 500
Size Maximum Tonnage := 500
Num Must Have At Least 1 Ability := 1
Must Have Ability 1 := Colonize Planet - Rock
Minimum Speed := 0
Desired Speed := 0
Majority Weapon Family Pick 1 := 0
Majority Weapon Family Pick 2 := 0
Majority Weapon Family Pick 3 := 0
Majority Weapon Family Pick 4 := 0
Majority Weapon Family Pick 5 := 0
Secondary Weapon Family Pick 1 := 0
Secondary Weapon Family Pick 2 := 0
Secondary Weapon Family Pick 3 := 0
Secondary Weapon Family Pick 4 := 0
Secondary Weapon Family Pick 5 := 0
Shields Spaces Per One := 0
Armor Spaces Per One := 0
Majority Comp Spaces Per One := 10000
Majority Comp Ability := Colonize Planet - Rock
Secondary Comp Spaces Per One := 0
Secondary Comp Ability := 0
Num Misc Abilities := 3
Misc Ability 1 Name := AI Tag 01
Misc Ability 1 Spaces Per One := 100
Misc Ability 2 Name := Standard Ship Movement
Misc Ability 2 Spaces Per One := 100
Misc Ability 3 Name := AI Tag 10
Misc Ability 3 Spaces Per One := 10000
AI Tag 01 refers to a 5kT supply reactor. It should add 5 of these (25kT), plus 5 5kT engines (25kT), and add AI Tag 10 for a cargo hold (20kT). The ship vehiclesize.txt entry requires 1 bridge, life support, crew quarters (30kT). So that's 100kT + 400kT colony module for a 500kT ship. It's a perfectly legal design I can make as a player.
Regardless of the ship size limits I set or even if I drop the colony requirements no luck to even get a basic hull design.
Strangely my attack ship entries are just fine!