Question about editing terrain

Is it possible to select an area smaller than 16sq M when editing terrain? I'm trying to create a small creek bed, but 16sq is just too large for parts of it, and I'm just making a mess of my land.

No, you cannot select an area smaller than 16 m2 using the select feature. If you are very careful with the editor and use the "small" setting, you can frequently get the desired results, but it takes a few passes and light touch. Sometimes you just have to hide spots that you can't make work by using rocks or plants.

That's right. Try clicking once at a time instead of holding the mouse button. Easy does it.

Another trick that I use a lot is to dig the ditch (as it were) using "lower land" with small setting on the land edit tool. If the ditch is too wide, change your setting to "flatten land" and sweep very gently from the sides to close in the ditch. The "flatten land" setting will flatten starting at the height where you first click with the tool, so make sure you are a little bit away from the edge of the ditch and then flatten towards it. Go slowly, almost like you are feathering the strokes. It takes some practice, but it does work. I do all of my walkways and creek beds using this technique.

Note that you can write a script (or find one) that adjusts land better than you can do it with the edit terrain tool.

Also, as others suggest, you have that 4x4 area, but you can move it 1m side to side.

Check out llModifyLand () in the scripting wiki, and you will see that the server allows up to three sized "brush", 2 x 2, 4 x 4 and 8 x8 M. Since land heights are specified in a 256 x 256 raw map, it would make sense to have a 1 x 1 "brush". This is discussed briefly in:
http://rpgstats.com/wiki/index.php?title=LlModifyLand

One of my alts (so I am not advertising here) has made a tractor that will travel a zig zag pattern over a parcel while carrying out one of the llModifyLand functions. Raise and Lower don't work very well that way, but Smooth and Level do work well.

There are also some very simple free scripts available that run the "level" command repeatedly while you move the object it is in with the prim editor over the land.

The idea of the brush is also misleading. A programmed "raise" will increase land height under the brush by 20 cm directly under the brush and by 10 cm under the same distance adjacent in all directions to the brush. So repeated applications in a row affect some parts disproportionately.

I am yet to see a clear explanation of the formula used in applying terraforming commands and the interpelation between the heights specified at the one meter points.