I use 177.5mms and I have the femur length to handle 180mms and I get appreciably more power than when I experimented with 172.5mm cranks.

Don't anybody jerk you off here - longer cranks do give you more power just like a longer crowbar let's you do more damage than a short one.

How about these modern extra-long golf drivers that let the Average Joe hit a golf ball 300 yards? The fact that a dwarf like Pantani used them for climbing illustrates the advantage. BTW, Miguel Indurain used 190mms in his 1 hour record.