Brand New, and Questions

Hi everyone, I am thinking about buying an aneros product.
I currently have a medium sized butt plug. Measures about 5 inches of insertion, and 1.75 inches at its widest. It gets in fairly easily.
http://www.stagshop.com/product.php?productid=55034&cat=462779&page=1
So I'm not sure if 'size' is an issue with choosing an aneros. I've read lots of statements regarding 'mobility' and 'fullness'. I'm interested in knowing what's important, or how it's supposed to work, and why this mobility is important. I'm not a 'size king', and really have no desire to make my rectum all loosey goosey if any of these products are designed to 'push the limits. (I read that gay guys prefer the bigger ones, and I'm straight, so I was confused about some of the purpose of the 'bigger' ones.)

It would be nice to be able to sit down and lie down flat with the product as well, if anybody has any suggestions regarding this.

Also I did read about potential hazards, since the Aneros is designed by medical professionals, I assume it's fairly safe. But I have read how some models 'aggresively' attack the prostate and am not sure if that entails to the following article I found about prostate massage. So when reading about how the Helix/Maximus/Prograsm offer more pressure, I get weary.

"Other possible serious health dangers from prostate milking could be Fournier's gangrene, blood poisoning,the transfer of prostate cancer to other parts of the body or hemorrhoidal flare up.

One of the most important area's of concern in doing a prostate milking yourself is to apply a minimum amount of pressure. The maximum amount of pressure you should apply would be equal to the amount of pressure you would rub your eye with. Applying to much pressure during the prostate milking could lead to some of the above mentioned health dangers."

So I guess the bottom line, is are there any suggestions on which aneros I should get?

Thanks