Several options :

1. You have to constrain the DIV blocks. If you fix a width just for the black column, you'll still have a problem with the remaining one. To avoid fixed pixels widths you can constrain both as for example 70% to 30% relative width, with a "display:block" adjustment.
You'll have to set also a "min-height:, height:auto and height fixed width beacause a div blcok can behave inproprely concerning its lenght.

2. You can leave as is for the main divs and constrain a sub div block within the black column div using same method as written above.

3. Or you can fix width just for the elements contained within the black div (images / ul lists, etc..)