Welcome to aviation photography. First and foremost, let me say, you should take photos for you, not for A.net. A.net is a great place to display high quality photos but it shouldn't be your sole purpose for taking photos. You'll be mighty discouraged after several rejections (and you will have several rejections) if you're only taking photos for the purpose of uploading here.
Secondly, a D70 is a fine camera to produce high quality photos for A.net. When you say the photos are too big, that's because you're probably uploading the files right out of the camera. The photos out of your camera need to be edited down to be accepted on A.net. This should answer some of your questions:
Hope that helps and we look forward to seeing new photos from you!
[Edited 2012-12-28 06:12:09]