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:

Mk7 torpedoes when you are in sweeping range:

md1: Mines:     810 / Radius   28
md2: Mines:   1620 / Radius   40
md3: Mines:   2430 / Radius   49
md4: Mines:   3240 / Radius   57
md5: Mines:   4050 / Radius   64
md6: Mines:   4860 / Radius   70
md7: Mines:   5670 / Radius   75
md8: Mines:   6480 / Radius   81
md9: Mines:   7290 / Radius   85
md0: Mines:   8100 / Radius   90
mda: Mines: 22500 / Radius 150

Mk7 torpedoes with Minefield decay 5%:

md1: Mines:     770 / Radius   28 (-0)
md2: Mines:   1539 / Radius   39 (-1)
md3: Mines:   2309 / Radius   48 (-1)
md4: Mines:   3078 / Radius   55 (-2)
md5: Mines:   3848 / Radius   62 (-2)
md6: Mines:   4617 / Radius   68 (-2)
md7: Mines:   5387 / Radius   73 (-2)
md8: Mines:   6156 / Radius   78 (-3)
md9: Mines:   6926 / Radius   83 (-2)
md0: Mines:   7695 / Radius   88 (-2)
mda: Mines: 21375 / Radius 146 (-4)

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).