Select 2 edges using the wooden axe, then write /set 2.
If you want to add some natural tallgrass on top of something, select 2 points and write /overlay 15%tallgrass:1,85%air.
I've made a little guide on worldedit incase you wanna know some more things. http://www.youtube.com/watch?v=vDnZMNbn2OM
If you have installed Single Player Commands with WorldEdit, you can stand at the first corner and type "//pos1", then move to the second corner and type "//pos2".
Then you can type "//set 3". Three (3) is the ID for grass or it might be Two (2). I always have trouble getting the axe to work. You can also delete whole 3D areas by using "//set 0".
---------- Post added 2013-01-03 at 06:59 PM ----------
Oh, if you want tall grass, just hit the grass with some bonemeal.
For selections, you can use the wooden axe to leftclick a block for point 1, rightlick for point 2, and you can write //pos1 to select your position for point 1 and //pos2 for point 2, and you can also write //hpos1 to select the block you are looking at (it's like a long range wooden axe) and //hpos2, it really helps making a powertool of //hpos1 and //hpos2 (powertool is a part of essentials, you write /pt /hpos1 for example and you click with a block instead of pasting the command into the chat).
You can change the selection type with for example /sel cyl, the the selection becomes a cylinder instead of the standard cube selection.