The circumference is actually 3.5 inches. Which is actually a good beginners size. 
Its not the easiest shape for a first plug to be honest, they can be a bit tricky to insert if you havent done it before. It also has a wide neck which may have caused the discomfort. Most plugs have a smaller circumference neck which makes them more comfortable to have in. This doesnt and as a first plug its going to be more of a stretch
Just incase there was some confusion. You want to insert it so the smallest area goes in first, then turn it to continue inserting the rest.
Its also possible you clenched up on it whilst inserting it. Which is easy to do accidentally when you start out. Its really important to take deep slow breaths and make sure to relax your muscles and go slowly. Both whilst inserting and anytime your feel yourself tensing when you have a plug in
(I often find concentrating on how much you want a toy in can help too)
You also dont have to insert a plug in in one go and can gently nudge to tip against you anus untill you relax more. It can take a while so go slow and dont feel like you have to rush it. Listen to your body and if its uncomfortable take it out.
All else fails, return it and swap it for something of a different shape (these are both good options).
There are also good recommendations for prostate massagers in the p-spotting thread
If you want something thinner to start off, This one is smaller than the plug you bought.
This is a more traditional bulb shape and though the max circumference is the same as the plug you tried, it should be more comfortable to inset and wear as it tapers at the neck and tip