div {
    box-sizing: border-box;
}

.board {
    width: min(84vw, 84vh);
    margin: 10px auto;
    outline: 1px solid black;
}

.board .field {
    border: 1px solid black;
    width: min(12vw, 12vh);
    height: min(12vw, 12vh);
    display: flex;
    align-items: center;
    justify-content: center;
    float: left;
}

.board .row .field:first-child {
    clear: both;
}

.board .field .piece {
    width: 95%;
    height: 95%;
    border-radius: 50%;
}

.board .field .blue {
    background-color: blue;
}

.board .field .red {
    background-color: red;
}