Ummm, 40 acres is probably to much actually

http://www.engadget.com/2007/12/19/t...r-your-garage/

Additionally if a ships nuclear reactor needed 40 acres... woooo

But honestly to the OP, that is so damned super omfg level effective,
except one thing.

You said max length. Ooops! Of course that only saves you from sloppy
downards level work, but imo if someone was after you, this would now
give them the correct length to try instead of working their way up.
It is 7 bits larger than the one below (If we assume 128 outcomes, but
your not quite there) however the man now discounts all other size
standards, saving some brute force time. Still you are as close to
maximal security as can be created. Now I ask, why the hell do you
need this much security? lol