To steal an idea directly from Dead Space; IE Planet Crackers, a mining ship whose mission is to strip-mine an entire planet. With something like that, an invention I would expect of the human race, such trivialities as resources disappear, the only problem remaining is transport of the materials to a ship-yard and the building/manning of the ships.
Depending on what kind of FTL you're using, because you most likely are using some sort of it in these kind of games, I don't see very many battles taking place outside of a gravity well of some sort, the exception being if you ambush a well-established trade route, and even then, the odds are against you being able to successfully catch up to a ship that is moving at +.5c, let alone +c...
IMO the two main factors are A how many resources are available in your system, including anything the planet crackers bring in, and how industrially established is your system. You can have enough resources in your system to build 100 dreadnoughts, but if ship building is a cottage industry, you'll probably never build one, where as if you don't have the resources for one, but you're a industrial super-power, you'll probably be producing frigates for the other planets and using the profits to buy the raw materials for your fleet of 101 dreadnaughts.
There are anywhere between 200 and 400 billion stars in our galaxy, for the sake of argument I'll assume all of them have some sort of space trash floating around them. All you'd need for a 'quick' game of conquer the galaxy is about 100...
The fist dice I pulled outta my desk was a d20, so I'm going to make my imaginary resource determination system off of that. 2d20*1d20 gives an average amount of system minerals at about 220, meaning that it'll last for a while, but not forever. (the max per system is almost 4x that: 800)
The speed at which you can harvest and turn into a finished product is based entirely on how industrialized you are, a scale from 1-10, the downside being that the more industrialized you are, the more it costs to maintain the standard of living your people expect. Every turn you produce however many units as your lvl, and mine an additional lvl-5 point from your system, to a minimum of 0, IE a lvl 1 system produces 1 point a turn and subtacts 1 pnt from the system a turn, a lvl 10 system produces 10 but subtracts 15, it also costs the next lvl^2 to design, develop, and implement the next lvl of industry on your planet, lvl 2 cost 4 points and lvl 10 costs 100.
Planet crackers always produce half of the systems remaining minerals, and have minimal upkeep, but they can not produce anything on their own, they only boost your resources.
Ships have both an initial build cost and an upkeep cost, which includes the cost of sporadically re-equiping them with new weapon-systems and replacing broken equipment and staffing them.
Arbitrarily assigning numbers...
Dreadnaught: 50 build and 10 maintain
Frigate: 20 build 4 maintain
Transport: 10 build 2 maintain
Carrier: 20 build 4 maintain (hold 6 fighter squadrons)
Fighter Squadron: 5 build 1 maintain
Army: 2 build 2 maintain
Mech Squad: 5 build 1 maintain
Droptroops: 2 build 3 maintain
Planet Cracker: 50 build, 3 maintain
Of course all these numbers are BS, so please tear me a new one as you see fit...