What are you using right now?
256x256 http://www.minecraftforum.net/topic/127377-16x-32x-64x-128x-256x173-lb-photo-realism-major-gui-update-8252011/
256x256 http://www.minecraftforum.net/topic/127377-16x-32x-64x-128x-256x173-lb-photo-realism-major-gui-update-8252011/