Change width of Post/Page

You can, but you would need to be careful so you don`t break the layout.

Use the CSS plugin that I recommended on your previous question and add the following code:

@media only screen and (min-width: 1200px) {
	.content-area {
		width: 670px; /* Default value. Increase it as you need.*/
	}
}
@media only screen and (min-width: 1200px) {
	.site-content {
		width: 1070px; /* Default value. Increase it with the exact same amount you increased the previous value.*/
	}
}