Sorry in advance for a lot of information, but I'm assuming you live in the States, and won't be able to get to a store to try on in person...
Check the measurements on the Danier site. I thought someone reported, some time back, that because the apparel is cut and sewn by hands, there may be some discrepancy in sizing.
I tried on a bunch of leather jackets this summer when I was in Canada. For reference and comparison, I'm 4'11", 30DDD chest, around 110 pounds, and I generally wear a XS/2 for tops (I don't wear button-down stuff), and a 2-4 in pants and skirts. I got an XXS for my Danier jacket. I LOVE my jacket. I wear it with everything, open or closed.
With some of the Danier jackets, the XS was a better fit across the chest and waist - but when I tried on, I was about five pounds heavier, and a solid size 4. I've lost some weight since. For XS, I found the sleeves were uniformly a little too long for me, and the shoulders were a little wide. I cannot recall if there were any petite sizes, or if I tried any on. I bought my jacket in the XXS (not a petite), because it looked great worn open. Now it fits great, zipped up. It stretched a little, but the big difference I see is how much softer and lovelier the leather has gotten, with wear. I also bought the Danier weatherproofing spray, and so far, my jacket still looks pristine
The customer service was fantastic at the Oshawa store, and the clothes were beautiful! I will definitely go back, on my next trip to Ontario.
ETA: Jacket weight: Mine is a refined sort of moto, kind of mid-weight. I've worn it indoors and outdoors - it's been versatile as a substitute for a blazer. I started wearing mine in mid-October, in Atlanta, and it's been great, for 35F to about 65F, but then, I run cold. We had a week where it dipped below 35F, and I switched to a longer leather coat.