You can customize the widths, paddings and margins of your custom theme. However, in a two column layout, the left column should have a minimum of 660px, as several elements (such as video embeds) expect that width to populate properly.
As a reference, the official themes use a width of 960px. The left column is 660px and the right column is usually 240px. Generally there is 15px of padding on both sides of both columns, but this can vary a bit from theme to theme. You can also disable the sidebar on any page so it uses full width.
The custom theme documentation and object reference guide, available in the right column of your custom theme page in the control panel, will provide many other helpful details as you build your custom theme.