Thanks for the questions. I hope this answer handles both for you. I am assuming you are referring to prostate milking, so your best bet is the larger and longer toy. The reason is leverage and control. So grab the 7 incher, and start exploring your prostate. Go slow at first, and once you get used to the sensations and know exactly where and how your prostate likes to be played with, you can experiment with other toys or play!
Small bullet vibes tend to not work unless you wish to just let it vibrate up against your prostate once inserted, then it is OK. Not much sensation otherwise. And if something goes in and you do not have a grip on it, you will be pushing it out into the toilet!