html,
body {
    height: 100vh;
    width: 100vw;
}
body {
    display: flex;
    align-items: center;
    padding-top: 40px;
    padding-bottom: 40px;
}
.form-sign-in {
    width: 100%;
    max-width: 320px;
    margin: auto;
}
.form-sign-in > img {
    width:256px;
    height:256px;
}

@media screen and (max-width: 500px) {
    .form-sign-in {
        max-width: 280px;
    }
}
.form-sign-in .form-control {
    position: relative;
    box-sizing: border-box;
    height: auto;
    font-size: 1rem;
}