Open a new window, 300 x 300 with a white background.
Using the line tool, draw a line across the window, leaving plenty of room between the line and sides of the window. Don't worry about making the line as long as a bar.... we'll be stretching it out later.
Click on the line with the magic wand.
Click on IMAGES/SPECIAL EFFECTS/ADD DROPSHADOW...
Set the color attribute to your choice
Set the opacity to 255 and the BLUR to 36
Set the OFFSET VERTICAL to 0 (zero)
Set the OFFSET HORIZONTAL to 0 (zero)
Click ok
Activate the magic wand
Right mouse click on the window to clear the old selection
Left mouse click on the window to make new selection
(At this point, you'll notice that not only is your line selected, but the entire window has been selected).
Click on SELECTIONS/INVERT to select only the line.
repeat the shadow process
Repeat the selection/invert/ and shadow process again.
RESIZE the window by clicking on IMAGE/RESIZE
Select custom size
Set the line length to 500, the width to 30 Unclick maintain aspect ratio.
Click ok
If you crop the picture close to the image prior to resizing, the bar will have different dimensions.
Variations
This is the same image, sized to 500 x 50
Try using the find horizontal edges filter! This particular image doesn't convert to a transparency well, and would need to be used on a black page.
Use the gradient fill tool on the center line before you add the drop shadows for this effect.