JSFiddle - React, Tailwind, and code Playground

HTML

<div class="container">
    <div class="menu">Menu (200px wide)</div>
    <div class="content">Content div. This div has max-width: 800px; min-width 200px. It has enough text that it expands to its max-width if there is space available to do so.</div>
</div>

CSS

.container {
    margin: 0 auto; /* to horizontally center the container. remove it if you want. */
    max-width: 1000px; /* your self defined 800px max-width for the content-div + 200px from the .menu's width */
    min-width: 200px;
}

.menu,
.content {
    height: 200px;
}

.menu {
    float: left;
    width: 200px;
    background: grey;
}

.content {
    margin-left: 200px;
    background: lightgrey;
}

@media (max-width : 400px) {
    .menu {
        float: none;
        width: auto;
    }

    .content {
        margin-left: 0;
    }
}