Snakey:

There is no best size, 'cos different people want them for different uses:-)

Some ground rules.
Make your texture sizes in powers of 2 (256, 512, 1024, 2048, 4096....)
The reason for this has to do with the way that computers work. Using sizes (especially in the X dimension) results in wasted memory, since just about every application will pad the size with unused bytes to the next up pwer of 2 so as to be able to make use of the fast built-in data-transformation functions. So 512x512 = OK. 513x512 = no-no ('cos it'll take up at least 1024x512, and possibly 1024x1024 when in memory) (and that's just in pixels. Since each pixel takes up a minimum of4 bytes - that's a lot of memory going to waste.).

Tileable images. You obviously now know how to do this from the example you gave, but that example would have very limited use since the tiling pattern would very quickly become obvious. Possibly not a problem for games, but unacceptable for still pics. To get best results for tiling, you need to get a photo of large area of wall and tile that. The repetitions will still probably show up but would be much less noticable..

Cheers,
Diolma