I generally use size 4 missiles on all fleet/fighters up to the Battlecruiser size who use size 6, i've never built bigger missiles - but then I've never actually built anything bigger than a battlecruiser - which in my game is 24kton. Maintenance is 2-3 years (aiming for two minimum in the design) - fleets spend much of their time at places with adequate maintenance bays - be it colonies, outposts or shipping, it will be a nodal point covering a sector of systems usually no more than 4 jump ponts away, altho as always it depends on the game/map/xeno activity etc. Recon and patrols is usually conducted by specialist ships with hefty sensor suites and a decent speed (usually fastest in the fleet) but again with 2-3 years maintenance, each of my fleets will have easily half a dozen recon ships with a couple attached, a couple out on patrol and perhaps a couple in overhaul
Sizewize - and this is just my take on it:
Corvette/Sloop [1-3kton]
Frigate [2-5kton]
Destroyer [5-8kton]
Light cruiser [8-12kton]
Cruiser [12-15kton]
Heavy Cruiser [15-20kton]
Battlecruiser [20+kton]
I have never built anything bigger in a warship than 24 or 25kton, but if I did next classes would be:
Battleship (historically just a slower more heavily armed battlecruiser - i take this the same way)
Dreadnaught (light, standard and heavy versions depending on tonnage)
Monitor (this is the Starfire designation, which i like)
As always as size markers are not reliant on your actual size its always fluid, a more heavily armed and armoured 3kton ship may be classed a light destroyer whereas a speedier lighter 5kton ship next to it is classed a frigate. I'd go with what sounds cool to you.