This script (not sure who originally wrote it, the author isn't specified) makes a text input field to specify the height in pixels you want to resize the HIT iframe to, and a button to apply the change. These are inserted into the same cell of the HIT header info table as the "Requester:" label. Due to the width of the text field and button as originally implemented, and the limited width of the "Requester:" cell, the button unnecessarily wrapped to an additional line for me. I modified the script to eliminate this minor annoyance.
This script does not automatically resize subsequent HITs; you have to choose to click its button on each HIT that you want resized.
Simultaneous usage of other scripts that affect the HIT iframe size may interfere with this script.
See also: my MTG thread post
For many more MTurk-related userscripts, see the Turking Scripts set