Like Coot says, not floating that left element will let it take what space is available. It will not let it drop down to the next line like you want, but since #container has a min-width setting that doesn't much matter.
Some other things he changed are the width and height settings you had on #container. You don't need width: auto; because you are specifying widths with your min/max. Auto is default when you don't specify so it's not even needed then, unless the element is inheriting and you want it to not.
The height setting you had limits the element to that 600px. You already cleared your floats with overflow: auto; so #container encloses it's contents, a height setting now will just cause trouble later when you increase the size of a float.
Look how one element can drop here -
margin: 0px auto;
border: 1px solid #ff0000;
margin: 0 0 0 10px;
border: 1px solid #5882fa;
border: 1px solid #cacaca;
<!-- end #container --></div>