Welcome to the forum.
The Dixie Gunworks catalog says the Pedersoli rifles use a nipple with a 1/4-28 thread.
Before you get up and run out the door, Dixie also says my Pedersoli Tryon halfstock rifle also used a 1/4-28 thread but wanting to varify this I removed the nipple from my Tryon.
It is a 6mm X .75mm nipple thread just like most of the other Italian reproduction rifles.
With this apparent confusion my advice is for you to remove the nipple and take it to a good Hardware store or a Auto Supply and ask them what the thread size is.
That way you will know for sure.
I don't know what your background with muzzleloaders is so I'll mention that with most of the thread sizes available there are nipples made for Cap & Ball revolvers and nipples made for rifles.
They are different so when you order, make sure you are ordering the rifle sized nipple.
Here's a link to Track of the Wolf.
They are an excellent company with a fast turnaround time and reasonable prices.
http://www.trackofthewolf.com/List/Item.aspx/807/2