Yes, using custom CSS. The theme uses id’s to dynamically adjust the width of each column, depending on page type. The left, center, and right columns are labeled with the following id’s respectively: #sidebar, #primary, and #secondary. Here is the CSS that controls column width:

#sidebar { width: 200px; }
#primary { width:669px; border-left: 1px solid; }
#index #primary { width: 438px; border-right: 1px solid; }
#permalink #primary { width: 538px; border-right: 1px solid; }
#secondary { width: 0; display:none }
#index #secondary { width: 230px; display:block; }
#permalink #secondary { width: 130px; display:block; }

This gives three possible layouts: All three columns (e.g. home page or tag index); all three columns, but with a wider center column and narrower right column (e.g. permalink pages or ask page); two columns, left and center only (e.g. pages).

Column widths may be adjusted by tweaking the styles via custom CSS, however please note that the theme uses fixed sizes for photo, photoset, and video posts to correspond with the default column widths. Adjusting those requires tweaking the appropriate theme tags.

As for font sizes, custom CSS can again be used to adjust the default settings. Each column may be individually targeted using the #sidebar, #primary, and #secondary selectors, as well as individual elements using more specific selectors.