When searching in Framework 6.0.x versions, the width of the search window is dependent on the size of the largest item in the window. With Documents especially, this can cause very large search windows depending on the total path size of the documents.
In the theme for the portal, the size the Search pane expands to can be set to a maximum. To do that, go into the Theme's CSS file, and add the following:
The attached file to this case will set the max-width at 25% of the width of the screen. It should be dropped inside the theme folder you are currently using within the App_Themes folder.