You can use either Pixel or Percent widths with either Panels in a Panel Set or Columns in a Field Editor. By default, Pixel width is assumed. To use Percent widths, you must explicitly add "%" after the number you use, e.g. "50%", "25%".
With Panels in a Panel Set, the trick is that if you want percentage widths, you need to set the Panel Set Type property to "Custom Width".