The "row" width in Foundation is set by default to 1000px. At widths greater than that you would need to do a media query and use css to set the parent row max-width to 100% and then set the panel's width to 100%.
CSS
@media only screen and (min-width: 1001px) {
#panel-div {
max-width:100%;
};
#large-panel {
width:100%;
}
}
HTML
<div class="row" id="panel-div">
<div class="panel" id="large-panel">panel goes here</div>
</div>