Your css is not correct, you are looking for an element with the class ms-webpart-zonethat has a child element with the class ms-fullWidth which is not the case in the markup you show.
Try this:
<style type="text/css">
.ms-webpart-zone.ms-fullWidth
{
width: 700px;
}
</style>
Or, to be compliant with all browsers (some do not like to combine two classes like I do above):
<style type="text/css">
.ms-fullWidth
{
width: 700px;
}
</style>
or
<style type="text/css">
.ms-webpart-zone
{
width: 700px;
}
</style>
You will have to check yourself which one of the two latest variants that works the best on your specific page.