As it is only slightly thicker than normal civilian apparel, the APJ™ can be worn under any clothes, regardless of how thin it is. Also, it can be worn in the open as covert armor as it perfectly resembles casual clothes.
You will need to remove the ballistic panels, as they are not washable. Once they are out, it is completely possible to machine-wash the shirt and dry it as any regular clothes. Return the ballistic panel inside only once the shirt is dry.
If you need ballistic protection regularly, it would be prudent to buy multiple shirts and a single ballistic panel. That way you can wear a new clean shirt every day while not needing a dedicated panel for each one.
By measuring your chest and waist.
Your perfect fit is a combination of the size of the plate and the cummerbund. For that, you will need to measure your chest and waist and can then get an ideal combination.
At the top of the page, there is Size Chart where you can select the option Determine Size, which will tell you exactly which size will fit your requirements perfectly. You can also check the Size Chart at the Tech Specs.
If you want to stay covert while adding protection against rifles, you will need to use a low-profile plate carrier like the WPC™ Weightless Plate Carrier. In such a combination, you can still wear almost any type of clothes over your armor without it being noticeable.