The purpose of this library object is that you can place it as-is on the page when something is selected on the page, so it will be replaced with the library object.

To create a fixed size object

In order to ensure that the library object will not be resized when replacing an object on a page (and the page object has different dimensions than a library object), specify Fixed in Width and Height in the Tool palette for that object.

To create a fixed height object

If you want to keep only the height fixed, and you want to allow the width to extend to the width of the replaced object, specify Proportional in Width and Fixed in Height in the Tool palette for that object.

A special attention is to be paid if the object is part of multi-element library object, and it's not in the top left corner. For an example, see the data for the caption element in Examples of Tool for blocks with headline, body frames, image and caption space.