I've got 8 foot medium heavy rods for my downplaners and 7 foot medium heavy rods for my downriggers.
I had a young lady a couple weeks ago who caught a nice king on the 8' downplaner rod, it tripped the deep6 downplaner but she just didnt have enough arm strength to do the pull-up-and-reel-down thing against the fish, the residual motion of boat, the drag of the flasher, and the drag of the planer.
I didnt realize she was having troubles until I noticed that she had the rod tip pointed straight at the fish and was just cranking him in.
I started encouraging her "Pull up and reel down!" & "Keep tension on the line so he doesnt spit the hook!" and she angrily snapped back "I CAN'T!". She got the fish in finally, but she was plumb wore out by the time we boated her fish (and then she was all smiles).
It got me to thinking that maybe a shorter 6'-6" pole would be better for the downplaner rods since the mechanical advantage is all to the fish and the gear drag with the longer poles.
I went with the 8 foot poles to get the lines up and back a bit from the lines/rods in the downriggers but the planer lines go back at such a shallow angle it really doesnt matter much how long the poles are.