Hi. That's not really a Grav question but a CSS question.
If you are using Grav, there may be a simple answer depending on the theme you are using. Please say which one.
With the small amount of context you provide, I can suggest looking at which HTML element contains your content you want to limit. The CSS property called max-width could be set to either a percentage or a fixed value on devices with screen width greater than X.
There's a whole lot more to this and several other ways to achieve the same thing, but it's too much to explain everything here and I don't even know what you already know.