On this bike it's a 4-4 system so no crossovers or merges. It's 1 3/8 od at the head stepping up to 1.5" od after the first bend. It'll stay that size all the way to the mufflers.
When I had the 2-1 on my XS the headers were 1.5 od, stepped up to 1.625 od, merged into 2" to the muffler.