Caixa centrada e fixa.

body {
text-align: center;
margin: 0px;
padding: 100px;
font-family: Georgia, "Times New Roman", Times, serif;
font-size: 18px;
}
#caixa {
background-color: #EEE;
text-align: left;
width: 650px;
height: 550px;
margin-right: auto;
margin-left: auto;
padding: 10px;
}