Sector Gouverneurs give 1/4th of their bonus to planets in the sector, iirc. But I can't remember the formula for sector size. At some point it is just rediculusly expensive if you want the radius to increase.
I think the radius is specifically systemwide, with each increment being 1 jump further from the system of the command. The radius increases as you build additional sector commands in the same place.
Oh, I mean any tips on finding them? My fighters should be able to deal with them but I would have to go to every system that I previously noticed precursors in and have my fighters patrol around the outer edge of my active sensor range. Even if my sensors are off they seem to keep away from me. My longest range sensor is for 10kton and these ships are like 120-150 thermal craft that have fairly short range active sensors. My game is grinding to a halt because I'm running 12 minimum intervals at 30 days and only covering ~14 days at a time, when all enemy ships in the known galaxy have been destroyed.
Build/move Deep Space Listening Stations onto a single body in each system you're suspect of getting contacts. Stick them all on only one or two bodies in each system, as their range scales with how many are on the same body, so you can get arbitrary thermal AND EM range from system bodies without having to retool shipyards or somesuch, as long as you got the freight to move it. I stick DSTS's on a body near the jump points in almost every system I find, as soon as I can, just so I can have much more reconnaissance on potential enemy movements, especially including systems I don't even have a presence in, even hostile systems sometimes just because of how low-profile they are for the sheer sensor strength they're packing. Just be careful though, a geosurvey vessel may be able to find them for your enemies if they happen across it, and they may be glassed or captured, but it's still probably worth it to set 'em up anyway.