Standard sizes are around 512x512(gaming mostly) to 1024x1024(alphas and panaramic).

1024x1024 is the best, IMO.

Tileable and seamless

Be sure to include choices: Bmp's for best quality PC and Tiff for MAC users.  jpg or Gif for those trying to save memory space.

24bit color rez at 300 dpi.(measured as Dots Per Inch)

 Also keep in mind, that the new Intel mobo and Nvidia Gcard will allow 64 bit resolutions.

So maybe, the textures of the near future, will be even more detailed.