After my interest was piqued in-store, I ordered this blazer online (also available in solid red, navy, and navy stripe) and have since been very pleased. The material is stretchy and super comfortable, plus the working pockets are a nice touch. If you are taller than ~ 5'1" and do not want a "shrunken fit" blazer, I would recommend the regular sizing over the petite. I tried on a regular XS in this post and think that length looks better if you usually wear jeans or pants. I kept the petite since I'm most often in skirts and dresses, which I've learned look more flattering with shorter jackets.
In response to a few requests, I'm working on a "staple series" of posts featuring items that have been the foundation of my wardrobe. A ponte/knit blazer is definitely on that list - I wear my striped and solid ones on heavy rotation, plus use this as a "lining" inside coats on especially cold days. If you don't already own a versatile navy and white striped version, I'd recommend this jacket in that pattern.
^ Jacket back view. Aside from a visible torso length difference of 2-3" and sleeve length of ~1" (when uncuffed, the sleeves here are "bracelet length" on me) between the regular and petite sizing, I unfortunately can't speak with certainty about differences in width. I remember the regular sizing fit fine but it was over a sweatshirt I was also trying on, so I'm not sure how it'd look with a regular, thinner top.
Measurements of the petite XS: 15.5" across the armpits, 14.5" across the waist when buttoned, 13.25" across shoulders, 19.5" back - 20.5" front torso length, 20.25" sleeve length. Stretchy 85% cotton, 15% polyester, unlined.
The simplest rule to remember when mixing patterns is to use prints in contrasting sizes. Stick to a palette of 1-2 colors if you want to be safe, and layer with neutrals. Boden does this well - their spring catalog is full of gorgeous print-mixing inspiration!