You can specify the column widths when creating them. Ater that, users may resize the FlexGrid columns using the mouse. As in Excel, they may drag the column header's right edge, or double-click the edge to auto-size the the column so it fits its content.
If you want to prevent users from resizing the columns, you can either set the grid's allowResizing property to 'None', or prevent them from resizing specific columns by setting the column's allowResizing property to 'false'.
For example, the grid below does not allow resizing the 'ID' column. Other columns may be resized and auto-sized: