The Nu Minefield preview is in a way incomplete, so i end up drawing circles all the time.
And since i don’t wish to calclulate each minefield, here are the most common sizes:

Mk8 torpedoes when you are in sweeping range:

md1: Mines:   1000 / Radius   32
md2: Mines:   2000 / Radius   45
md3: Mines:   3000 / Radius   55
md4: Mines:   4000 / Radius   63
md5: Mines:   5000 / Radius   71
md6: Mines:   6000 / Radius   78
md7: Mines:   7000 / Radius   84
md8: Mines:   8000 / Radius   90
md9: Mines:   9000 / Radius   95
md0: Mines: 10000 / Radius  100
mda: Mines: 22500 / Radius 150

Interesting: The biggest minefield one will ever see, will have a radius of 146 lightyears.
When laying a maximum size minefield that no one sweeps, the decay of five percent will destroy 1125 mines, which is the equivalent of 14 mk7 torpedoes or the sweeping power of 3 heavy phasers (webs: 4).