What do you mean by EM Sensor (80-100)?
I can't open my software because I'm at work, but I thought the sensors scale to 50 hull spaces as the upper limit. Could be wrong.
Size 25sensors should be more than sufficient at moderate tech levels to do a passive survey of most of a system. I typically prefer thermal sensors of the same size, because I'm more interested in finding the precursors and other hostile vessels. I wouldn't typically want to go more than 25, which is a big honking sensor, for a ship. Just too big to go 50, especially if you want both types. These big honking sensors eat a lot of maintenance supplies when they pop too, so these on-station sensor platforms need a lot of maintenance to keep them from breaking down.
Once I buff up my EM and thermal sensor tech enough, I'll create a stealthy ship (high thermal signature reduction, cloaking device if possible) with big honking sensors on it like those. Its job is to jump into a system and hang out and monitor things. I've found I can usually monitor the jump zones from roughly the position of the central star, but I haven't tried to do this with starter tech.
Once you build a design, you can test it out by checking the sensor tab. You show passive sensor ranges, then set the size of the signature. I think most ships I encounter are in the range of 1000, so I'm able to observe them from quite long ranges.