I want to build the Picnic table with built-in cooler in a 12' length. The Home Depot "How To" is for a 6'. Any ideas on how to do this? Here is the link:http://www.homedepot.com/c/how_to_build_a_picnic_table_with_built_in_cooler_HT_PG_LC
Spanning almost 12' with 2x lumber is going to result in the table that wants to sag. Basically you would have to add a leg assembly in the middle just like the ones at either end. The "cooler" is going to be in the way if you leave it centered. I would suggest making two coolers, one on either side of the center support.
Frankly it may be much easier to build two tables and just butt them together for use. A 12' table is going to be really heavy and difficult to move.
Remember to use wood and fasteners suitable for the outdoors so your table lasts for years!