The recommended way of specifying column widths is to set the WidthType to Scaleable which allows the column widths to be specified as a percentage of the width of the entire list. So for example a column with a Width of 20 would take up one fifth of the entire list width. The last column in a list should be specified as WidthType Remainder in which case the column takes up whatever space is left unused.
The width of the columns can be adjusted visually by sizing the column headers in the design view. The value of the width property is adjusted accordingly.