In general, DD means daughter. Most people think of it as Darling Daughter, although there have been a few other interpretations as well. To move up from Jr. Member you have to do a certain number of posts. I'm not sure what the number is but it's something like 10 I think.
For the avatar, I believe the maximum size is 100 x 100 pixels.
For the avatar, I believe the maximum size is 100 x 100 pixels.