As far as hull choice goes, the hull size starts to matter less and less the faster your ships are. See viewtopic.php?f=21&t=3141 for details on the hit chance formula, but in short if your ship’s speed is greater than the tracking speed of the enemy fleet weapons, you are mostly immune to their fire.
Some terminology notes:
I consider fast frigates to be frigates whose speed is above 0.7-0.8 speed, while I consider frigates to be ‘slow’ if they have less than 0.3-0.4 speed. Mid-speed frigates are anything that doesn’t fit in either of those ranges. Other people may have different requirements for what they consider ‘fast’ or ‘slow’ frigates, but this is how I classify my frigates and what I mean when I refer to ‘fast’, ‘mid-speed’, or ‘slow’ frigates.
- Speed - I tend to prefer my fast frigates to be above 0.8 speed, and if possible I want them at 1.0 speed or higher (Parasites can reach 1.0 speed using two ion cannons on the Staurozoa hull, though I’m not sure if they can do so on any of their other frigate hulls). If I’m going to sacrifice firepower for speed, I will usually first switch the Ion Cannons for Rapid-Fire Lasers, since those are lighter and have much lower power requirements, with each one having about 70% of the firepower of an Ion Cannon and a much shorter minimum range (unfortunately, it also comes with a higher crew requirement, lower tracking speed, and lower maximum range, but higher speed can make up for the range loss). If I want more speed than that gives me, then I’ll consider dropping from two turrets to one turret, although I’ll consider switching the Shield Generator II out for the Shield Generator before dropping weapons.
Most of the time, if I’m looking at adding a third weapon, I start to consider the frigate a mid-speed frigate rather than a fast frigate, which means that I either want to have lots of them or I want something to bring an EMP to the battle; lots of them and all of them carrying EMPs is also good. If I have the slots and budget available for it, mid-speed frigates will get a bit of armor (two plates of Frigate Powered Armor, if I can manage it, since that is usually enough for resistance to fighter rockets). Nevertheless, there are a handful of frigate designs which fit into my fast frigate category which also carry three weapons. Parasites can make a 0.95 speed Staurozoa with three Ion Cannons for about 1400 credits; the speed 1.0 two ion cannon Staurozoa costs about 1000 credits. Whether the extra Ion Cannon is worth that much is entirely your decision. Using Rapid-Fire Lasers or mixed Ion Cannons and Rapid-Fire Lasers can also produce three weapon fast frigates.
Dropping everything except a shield generator is normal for fast frigates. Slow frigates tend to get at least enough armor to protect them from fighter rockets, and will carry more than that if there is available space and budget, unless I’m only taking the frigates for the extra guns (usually only if I’m deploying Imperial Weapons Platforms). Mid-speed frigates will get whatever I deem good enough - if there are lots of rocket fighters I might try for 12 average armor, otherwise I might give them a second shield. I tend to put EMP missiles on mid-speed frigates, since that tends to protect them better than any amount of armor or shielding I could give them.
As mentioned at the top, size matters mostly on the low end of the speed scale. However, it also matters when you deploy different types of frigate with similar speeds, as the targeting AI shoots at what it has the best chance to hit. Thus, you can help convince the AI to shoot at particular ships in your fleet - as long as everything is in range, slow ships will be targeted before fast ships, and large ships will be targeted before small ships.