TTG CE3 Viewport Sizer is a free utility which allows you to measure the size of the Lightroom Web preview in pixels. Useful for testing your galleries' appearance at various display sizes, or when designing Flash galleries for a specific embed size.
TTG CE3 Viewport Sizer has no options. Just load it, resize your Lightroom window until you have achieved the desired dimensions, then either select another web engine from the Layout Style list or Template Browser, or trigger an Undo to return to your previous work.
You can easily effect the size of the preview area by:
After setting your preview area to the desired size, use the 'F' key to easily switch between your full-screen workspace and the smaller, “actual size” view that you've measured.
TTG CE3 Viewport Sizer doesn't just run in Lightroom. You can export it to run it in your browser, either from your desktop, or upload it and use it online.
I keep a copy online at http://test.theturninggate.net/measure/ so that I can easily test screen resolutions whenever I get my hands on a new phone, tablet or computer. As a web-designer, I like to know such things.