I am a bit torn about the best size for mines. The larger the mine, the better sensors you can have, the longer the range it can engage at. But the larger it is, the more easily it can be detected in turn, and potentially taken out before it can launch.
20 is a good size for a light mine. 50 or so is the better for more standard mines. You can get way with having a small mine only firing one missile, like the CAPTOR mine. Even the largest missile you could make is still the size of a very small fighter.
So what range do mines need? Their most notable use is as warp point interdiction. So they need enough range to cover against the enemy jump engine range, and possibly a TL up from that.
Warp point defense, you only need a couple million or so as the best way to use them is to seed a ring around the point instead of all directly on it. For a general purpose mine you can use anywhere, 30 million to 50 million. For system defense (where you want to mine a whole system for whatever reason) you would really want 120 million+.
I could see mines being used as a forward defense of a warp point, where they are used to thicken the medium ranged firepower of the defense, before beam range. The theory being that the defense will use point defense against long ranged bombardment, will duck out through the warp point if there is a truly large missile wave, trusting to fighters staying behind on the warp point to whittle through the loitering missiles, while the mines make it hazardous for the attackers to move in while the defending fleet is on the other side of the warp point.
I use them in a few ways. 1) To detect enemies and defend warp points so I don't have to keep ships out by the point. 2) Seed a contested or owned system to protect against mass invasion fleets. 3) A defensive system for my scout ships. 4) Infiltrate an enemy system or travel route and lay mines to mess with them.
Actually, if defenders continually cycle part of their fleet through the warp point, they are likely to cause long range missile bombardments to lose lock outside of the range of their onboard sensors. They still go to their target's last location, but if that was offset from the warp point, the myopic sensors on the missiles might not be able to reacquire.
My dedicated mine missiles/torpedoes usually have a sensor range of 1 million or so. But the enemies I usually find use 15k ton ships as a standard so I can afford to increase the sensor resolution to 200-300 to keep the sensor small. Remember that mines are meant to fire without the aid of support, so they need to be designed to handle those scenarios.
