line=

  line=channel,NUMBER,START,WIDTH,STEP      (integers)
  line=velocity,NUMBER,START,WIDTH,STEP     (km/s)
  line=wide,NUMBER,START,WIDTH,STEP         (integers)

NUMBER

= number of channels to output

START

= starting channel number

WIDTH

= number of channels to average together

STEP

= channel increment

The mirhelp line command also provides more details (look for line.kdoc)