You want the table to be sturdy and all that, but the worst tables I have ever sat on were the ones where they sat so high my feet wouldn't reach the ground, and I'm 5'10".
Make sure you take some measurements of tables you have sat at that were comfortable and compare them to the measurements of the plan you choose before you start cutting and nailing. You will save a lot of heartache.
You could also just take measurements off a picnic table you like (go to the park, there are lots of them there...)and build your own from the measurements alone. I find I get a lot more satisfaction in figuring out how to do it myself than to follow someone else's recipe.