Well, the fact that the class in question is not used in the CSS, yet, doesn't mean it could not be added. So you could add custom CSS like this:
.hikashop_product_page {
max-width: 800px;
}
But then I'm not sure this is going to get you anything. Not only do you seem to have some in-line CSS added (according to the HTML in the screenshot on what you "need to do", maybe by some view edition or whatever), but - no offence intended, but quite frankly - your site is so screwed up with various different layouts and appearances throughout categories and products (none of which has to do with HikaShop), that neither I would know what to begin with to fix it, nor is this forum the right one to do site design and style for you. So, if you're not sure what you're doing there, I'd recommend you to seek help from a professional frontend web designer.