There are two ways you can go:
First one: Use a mediaquery
@media (max-width: 500px) {
#c7form_left {
float: none;
}
}
or use a fixed width instead percentual one. This wrap the columns automatically when scaled down:
#c7form_left {
width: 440px;
float: left;
margin-right: 3%;
}
Best,
René