Resizing on all screens

Site URL: https://lychee-hexagon-xtzb.squarespace.com/

Hello, I am very confused about Breaking points. 

I have a client who has a computer that's 4K and dimensions are 3840x2160

I am creating a site for him and the site looks good on a laptop but when it get's bigger it stretches out.

Unfortunately I can't see what he means as I don't have the same screen size as him so I don't feel like I am equipped to design a site for a larger screen.

I am learning more about coding and breaking points and I am wondering if there is anyone out there who knows a thing or two about how to have your content look the same across all screens.

I read an article about something like using px instead of pixels and ems or something like that. I have no idea it's uncharted territory for me.

Clients Site: https://lychee-hexagon-xtzb.squarespace.com/

Password: moretti

If I can't solve It he might hire another designer. So I appreciate the help!

As you are using a Squarespace 7.0 Brine-family template, there's an option in Site Styles that allows you to set a constrained width, so the site content does not get wider than your chosen width. This can be useful on some sites to prevent content 'spreading'.

You can read more in the support article Brine template family.