I find around 12KT-14KT to be a good blend of the various factors for most ships, except my carriers and small escorts. My carriers I tend to make as large as I can, and escorts as small as I can. My carriers are also usually my sensor platforms as well. Since they are the biggest ship, they tend to be targeted first anyways and are hardly stealthy, so why not put giant radars on them?
Carriers (20-30KT, sensor platform [with huge sensors for res100, res20, and res1] lots of fuel and ordnance, and ~10 fighters per10KT)
Missile Cruisers (~12-14K), only short range sensors for backup, dual role, long range missile strike and AMM platform
Destroyers (~12-14K), short range sensors, dual role, some beam for point defense, some beams for close range(jump point defense) ship to ship combat, sometimes I put on some ASM box launchers for alpha strikes
Escorts (~6k) short range sensors, point defense only
Late game, ill add hunter killers, small ships with high stealth for raiding, scouting and jump point interdiction. ~5K
Also late game, I may add (for fun/RP only, not really practical) a battle cruiser design which is 20KT-30KT and combines many roles. What might be called a "space control ship" from the old Starfleet Battles board game.