TYPO3 Core

Sorry, I hadn't read the manual:

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FAQ: Why are popup windows too small when they open?

We have tried to make sure that all popup windows open with usable dimensions. Since the contents of the windows may be variable and may be localized, it is difficult to ensure that all windows will open with the correct size in all languages. Therefore, the editor will try to adjust the width and height of the windows to their specific contents and to center the windows on the screen.

In some OS/browser configurations, the editor may be prevented from resizing and moving the windows due to lack of privileges:

• If you use a Firefox browser, select through the browser menu Tools -> Options -> Web Characteristics -> Javascript -> Advanced, and check the options that allows scripts to Hide the status bar and that allow scripts to resize and move windows. Popup windows should then open with the correct size.
• If you use IE6 on Windows XP SP2, select through the browser menu Tools -> Internet Options -> Security -> Trusted Sites, and add the url of TYPO3 site to the list of trusted sites. Popup windows should then open with the correct size.