Start soft , slow , sensual use also a good lube .
The best choice for you is medium size, small will be quickly forgetted in the depth of a drawer.
1"1/4 to 1"1/2 is a medium size in diameter , 3" is like Large or XLarge actually forget this sizes.
This toys is large or XL but you can use it from the tip to the first "ring" , monthes of exercises later the all toy will serve your pleasures
http://www.medicaltoys.com/anal-play.htm
Remember go SAFE , soft , slow , sensual , listen to your body , if your ass was sore for multiple days this means you was too wild and not cautius enough !
That last toy will make you scream your pleasures , if you not cautius enough you will scream your pain.
Play safe and mature adult