Guideliner allows you to measure the dimensions of your screen layout.

Optimize your layouts for screen and Web publishing with GuideLiner. You can measure your Web and screen layouts, check and optimize them for smaller screens and work with guidelines in all applications. GuideLiner offers horizontal and vertical screen rulers and guidelines which can be placed free on your screen (even overlaying other applications). Use these tools and effectively measure and proof on screen, if your layouts have the right size, etc.