rownum = LengthWhile[m, Length@# == 2 &]

2

m[[rownum, 2]]

6

If you need the last element in the second column you can also use:

TakeWhile[m, Length@# == 2 &][[-1, 2]]
6

rownum = LengthWhile[m, Length@# == 2 &]

2

m[[rownum, 2]]

6

If you need the last element in the second column you can also use:

TakeWhile[m, Length@# == 2 &][[-1, 2]]

6

rownum = LengthWhile[m, Length@# == 2 &]

2

m[[rownum, 2]]

6

rownum = LengthWhile[m, Length@# == 2 &]

2

m[[rownum, 2]]

6

If you need the last element in the second column you can also use:

TakeWhile[m, Length@# == 2 &][[-1, 2]]
6