Huge games are fun. We'll have to hook up a PBEM game once I get the program (have to check with my partner to see if I can afford it). A couple of hints from my experiences...HUGE settings delay your contact with the enemy. That's good because you have time to explore and to build your research/technology tree. In HUGE games I go for massive amounts of research points OVER material production. That allows me to build a LOT more tech very fast.
When I do encounter the enemy, I then recycle the research centers on mineral-rich planets...and re-build mineral processors there. That gives me a big boost in resources while reducing my research to more normal levels (balancing act there). Also, I manually control the recycling so I only get rid of one research center each turn a new mineral facility is built.
As far as ship construction/design, I go with a mixed approach. I use a 'balanced' design with decent engines, decent shields, decent firepower as my stand-alone units. These ships go into combat with individual or small groups of enemies and are designed to 'slug it out' enough that they at least heavily damage an enemy before blowing up. If I'm caught flat-footed without fleets ready to go, I push these puppies out to buy me time to build a real fleet.
When I have plenty of time to prepare because my 'balanced' ships are holding the line, I start designing and pushing out 'fleets'. These are ships designed to ONLY be fought together. If one of them gets caught alone or without ships of another type, they're usually dead meat. First off, the biggest ship I can build is designed to go as fast as possible (maximum engines, etc.) - this also gives it a better supply rating. Next they have the heaviest of armaments available with only the most basic of defenses (one shield generator, etc). They are the 'heavy punchers' designed to knock out enemy ships one by one until they are all gone. For every 'heavy punch' ship, I build two balanced ships of the next smaller hull size. They can take more of a beating and are used to stand between the heavy punchers and the enemy. If I started with a Battle Cruiser for heavy punch ship, the balanced ships will be the Cruiser size. Next, for every heavy punch ship built, I build three 'defensive' ships. These ships have minimal offensive armament, massive shields, and maximum defensive weapons (like point defense cannons). In this example they would be on a light-cruiser hull. They're meant to soak up as much enemy fire as possible, destroy incoming plasma torps or capital ship missiles, and take damage from the enemy while the bigger boys blow up the enemy.
Sometimes I'll also include frigates or destroyers with a few nice powerful offensive weapons to act as 'cleanup'...sneak in and finish off damaged enemy ships so the big boys can go after another target.
All fleets get at least one, but preferably two 'repair ships'. These are ships equipped with repair bays (best available) whose sole purpose is to FIX damaged ships after a battle. During battle, if a ship gets heavily damaged, I pull it off the front line and use other ships to try and save it. If the ship can survive the battle, the repair ships will patch it up and I have to spend less time re-building my fleets.
I'll have to see what the difference are between IV and V regarding shipbuilding, but in four I only put shipyards on planets capable of holding a population of 2 billion or more. Big planets holding 8 billion are best since the shipyards build faster with bigger population bases. Also, if they use the asteroid mining, don't hesitate to build asteroid miners. Now, don't deploy them until you need them (because they reduce the value of planet's asteroids as they mine). Wait until you're building a huge fleet and your resource collection is going negative...then deploy the fleet of miners you've built to every nearby asteroid/uninhabited planet with a high rating of resources. When you're done building or you've expanded enough, send the miners back to your nearest spaceport and mothball them (I usually keep 5-10 of these mothballed until I need them).