Medium or higher hulls

You get them by researching the Construction techs. Some of the Campaign missions don't have all techs available to research if that is relevant to what you are playing.

Tony

Thanks! Though Im not playing on the campaign...

The name of the Tech you need is actually called "Medium-sized Hulls". However, there's a pre-requisite Tech you need to research to get to it... I think it's called "Enhanced Ship Building" (or maybe just "Enhanced Construction"). This is the Tech that you need to build Tiny/Small Ship sizes. You may be able to already design/build Tiny/Small sized ships (without having researched the "Enhanced Ship Building") because your race allows for it. But, you still need to research it to move up the Tech tree (and reveal the "Medium-sized Hulls" Tech).

The techs you need to build medium sized ships are (in order) : Xeno Engineering - Basic Logistics - Advanced hulls - medium scale building. Going along that reasearch line will also eventually unlock large and massive hulls.

Yes you get them pursuing a certain branch of research. I can't remember exactly which one, but the tech to get medium hulls is called "Medium Scale Building" or something along those lines.

Edit:
I should refresh more while typing

I have the same issue with the game I'm playing right now. I have all the techs mentionned by Safari Joe, actually I also have advanced hull design, but still I can't design a medium-sized vessel. Or more to the point, I can't save the ships that I design. I will invariably get a message saying that I can't modify core ships. I don't have this problem if I design small or tiny ships.

Can anyone tell me what I'm doing wrong?

Use a differant name

Now any serious answer to my question will be gladly appreciated...

Hey pal, take a chill pill

The guy was trying to help you - use a different name for your design. You either haven't changed the name away from the core design, or inadvertantly picked a name that already exists as a core ship type!

I was serious if you try to name a ship design battleship or frigate or any other common name you get that message because it is a core design.  Even if you can't build it yet.