In design view, click on the field you want to change to select it.
With the Properties window open, look for the Width attribute (under the
Format or All tabs). Change the value to your choice.
If you mean: how do I change the width (number of characters) of a field in
a table when in design mode, then just change the value in the "field size"
property.
If you mean: how do I change the width of a control (that may or may not be
bound to a field) on a form/report, then the easiest method is to simply
drag one side of the control to the desired position.
Text boxes have a "width" property that can by edited; other controls like
list and/or combo-boxes have a "list and column width" properties as well.
Check boxes and radio buttons effectively have a fixed width.
-Ed
