<style type=’text/css’>
    body #s4-leftpanel {
        width: 300px;
    }
    .s4-ca {
        margin-left: 300px;
    }
</style>

Or you could use:

<style>
.TreeViewRememberScroll {
    WIDTH: 220px
}
.s4-ca {
    MARGIN-LEFT: 180px
}
#s4-leftpanel {
    WIDTH: 220px; MARGIN-LEFT: 15px
}</style>