What view are you using? Icons, List, or Columns?
In any view, go to the View menu and select Show Path Bar. It appears at the bottom, and usually there is enough room to display an entire file name if you click on the file.
In column view, Double-click on the || symbol at the bottom of the column, and it expands to the width of the longest file name.
In list view, double-click on the divider at the top right of the Name column, and it will expand to the width of the longest file name.