Each layout in a flexible content is wrapped in a div with a data attribute data-layout="NAME_OF_LAYOUT"
.
You can include a css file for the back-end and set your desired width for the layout like this:
.acf-flexible-content .layout[data-layout="NAME_OF_LAYOUT"] {
width: 500px; /* Your desired width */
max-width: 100%;
margin-left: auto;
margin-right: auto;
}