To accurately measure items on your screen you can use the free utility, Pixel Ruler. I've found this helpful when I have to upload a picture that's a specific measurement, such as a blog banner, or profile photo.
You can drag the ruler to any position on the screen, to line up the zero mark with the left edge of the object that you want to measure.
Then, point to the ruler where it lines up with the right edge of the object, to see the width in pixels. In the screen shot above, the rectangle is 223 pixels wide.
The ruler has three control buttons at its top left -- an arrow, a triangle and a line:
- To measure the height, rotate the ruler to vertical, by clicking the arrow button.
- Click the triangle button to change the ruler colour, or to access the Pixel Ruler web site
- To close the ruler, click the line button.