I always try to put on largest hull possible.

Usually put one set of armor on them (usually missiles for longest range). 

Then add all remaining space for more carrier modules (unless I need it to go a certain speed), but I usually use these to protect assets rather than hunt down things.

Late game I will use a Huge hull, with 1 transport module and build above as my  "Colony Warship" with lots of movement (usually 50-100 if not more). I then send packs of these out to take planets. 

The last reply is the most efficient, but do remember to limit yourself on the number of strike craft you're bringing to fights.  You can only take so many carriers before you're hindering yourself.  After a point additional ships don't get you anywhere and you begin weakening the power of your fleet.

There's a limit of 64 ships per side per engagement. Hull capacity spent on carrier modules beyond the number necessary to get [real ships in fleet] + [fighters in fleet] = 64 is, as a result, kind of wasted, though if I'm not mistaken the 'extra' fighters do allow the fleet to continue fielding an apparently-full fighter group despite losses for at least a little while.

I meant for subsequent engagements. I.e. you send in your carrier fleet of 14 ships which has, say, 100 fighters, so up to 50 fighters in the engagement, and the carrier fleet loses 30 fighters in the engagement. Next engagement you have 70 fighters in the fleet, which means you still get 50 fighters at the start of the fight, which makes it appear as though the fleet is still at full strength. Not sure if it still works that way, but it used to.

Some nice info here. I didn't know about the 64 ship limit.

FWIW I did try carriers based on cargo hulls in my last yor game, started with one module, and managed to cram in a second after researching/tech trading for capacity techs, still some had room for some defenses, engines etc. Those did well in combi with escorts and capitals.

Doesn't hurt to have armed cargo carrier designs just in case, if and when Frogboy overhauls the ship combat portion to have the AI actually take advantage of the ship roles feature. Defenses are pointless since carriers are already SOL if it's their turn to take hits, but weapons might conceivably be useful now, or in the future.

Besides, it's not like Interceptors can actually carry out the role of hunting Supports like they're supposedly intended to, even if the AI used them. If this ship guide is still current, Interceptors would have to take out the opposing Guardians... and then double back to wipe the opposing Escorts before they can proceed to finish off the opposing Supports. That means Guardians are obsolete in their intended role of protecting Supports, since Escorts can do so just as well and better. Yeah, I think that's kind of stupid too, but that's the game for you, what can you do.

For me, the key to using Guardians is what their targeting priorities are if there are no enemy Interceptors around; Assault. That means while Capitals might be preoccupied battering down the enemy Capitals or Escorts, Guardians might actually have a use in clearing out the enemy carrier Assault Fighters.

That's why I designed a Huge Guardian carrying 4 Guardian Drone modules; it's the best idea I can think of for countering carrier fighter swarms, while avoiding the overkill issues that Large/Huge Capitals/Escorts suffer dealing with them. Of course, it's still just pure theorycrafting, I haven't actually had the opportunity to test it out in practice yet.