How to set a fixed width column with CSS flexbox

The best way to achieve this is using the flex or flex-basis property rather than width.This will give you the best result in setting a fixed width column using CSS flexbox property. See the demo below:

.flexbox .red {
  flex: 0 0 25em;
}

The flex CSS property is a shorthand property specifying the ability of a flex item to alter its dimensions to fill available space. It contains:

flex-grow: 0;     /* do not grow   - initial value: 0 */
flex-shrink: 0;   /* do not shrink - initial value: 1 */
flex-basis: 25em; /* width/height  - initial value: auto