Huh? I'm not sure what you're asking... .dds files are whatever size you save them as... they don't have to be any size :huh:
[edit] Powers of two help, of course.
Many textures are 512x512. Open up a default Oblivion texture of whatever type you're doing in WTV.exe and copy it's size and format.
Read the teswiki texturing tutorials for more info.
http://cs.elderscrolls.com/constwiki/index.php/Category:Texturing_Tutorials.