I recently changed out the stock alloy 19" to a spoke 21" on my 2008 Xl883.
Still left plenty of clearance under the front guard there was no fitment issues, in fact I had the workshop lower the guard as a much as they could closer to the tyre there was still so much room left. The rolling diameter doesn't end up being a huge difference...
This photo is before I had the guard lowered to sit closer to the tyre:
Hope this helps!