Leather jacket break in period
 

I was at modern on the weekend and ordered a size 42 AGV Dragon leather jacket.

AGV Sport Dragon Leather Jacket - RevZilla

I've never owned a leather jacket so I was wondering if I should size down as it might break-in after a few weeks of riding and get bigger.

This happened with my gloves. Went from a Large to a Medium after they got really loose.

If a 42 fits just right should I just go with the 42 or size down to a 40?

from my experience none of my leather gear has ever broken in enough to warrant going down a size. at the very most i'd say my jacket stretched out about 5%. it's not as much about it getting bigger as it is about the leather conforming to the shape of your body. with that said, different brands/leather may be different in how much they stretch.