I wouldn't make too big a tub/tank since you'd need a lot of product to get coverage and would probably end up with more overall waste. If you want to go this route you could use the wing spar crate; put a moveable divider in it and line it with heavy plastic sheeting as needed for shorter or longer parts.
I use a standard plastic dishpan for small parts and a large (106 qt) clear plastic tub from the local Ace Hardware for larger parts. The large tub is made by Sterilite and will hold two main wing ribs side by side (measures 29" long by 16" wide at the bottom).
I fill either tub just enough to cover the parts (usually about 2") and recycle the used Alodine 1201 (dip with a plastic measuring cup and pour into the used product jug, followed by a funnel for the small amount that remains).
For larger parts (spars, etc.) I use a kitchen-size celulose sponge; while the coating is not as uniform as the coating on parts that have been submersed, this is not a problem according to the Technical Process Bulletin. See:
http://www.henkelaero.com/Images/Dat...ne_120_kit.pdf
Mike