Attribute for <MULTICOL ...>
WIDTH = integer

WIDTH controls the width of a single column. Every column is the same width... WIDTH sets that one width. The width can be expressed in pixels only, not in percentages.

WIDTH interacts with GUTTER, which controls the space between the columns. Generally it's better to use GUTTER to control the appearance of the columns, and not use WIDTH at all. Setting an absolute space between the columns controls the visual appearance far more than the width of the columns themselves.

Take a look at the page comparing several WIDTH values.