The 12 ft length has already been said. Because its a short kayak that a non-picky paddler will paddle, go with pine. WRC will only save a pound or two. Use a 1x4 or 1x6 and cut it into your peices. The 1x4 is actually 3/4 by 3.5 so 1x lumber is perfect for boat building. A 2x4 (1.5x3.5) is a pain to cut in half.
The big box stores usualy cary "select" lumber without knots, but most real lumber stores cary better quality lumber and have a pile that you can easily pick out your lumber. With that being said, I usually go to lowes/hd.
Scott