If you're on WordPress.com, yes—you'll need the Custom Design upgrade. Also note these forums here are for self-hosted blogs; for WordPress.com head over to http://forums.wordpress.com/ to get help from the community and experts there.
Width of columns can be tricky, it's probably not a simple change. Are you familiar with CSS enough to go in and change layouts? If not I'd recommend starting simpler since a layout change can be difficult to achieve without breaking other things (mobile layout, other pages, etc). If you are feeling brave, look in Reddle's stylesheet for:
/* Two columns, content on the left */ section—in there are the widths and percentages for the two columns.