Set it to fixed position, then resize it in /hudlayout. Click the placeholder once to select it, then press curl+home to cycle through sizes. Alternatively, after selecting the Action Help or Item Help placeholder, click the gear button in the central dialog to access element-specific options. Repeat for the other help type.

Note: there is a “tooltip size” option in /characterconfig, but this is not what you want. In the game’s terms, tool tips are just the name of the button that appears by your mouse pointer even when Help windows are in Fixed positions.