How to reproduce:
1. In KControl»Desktop»Window-Specific Settings make sure you have a policy that applies initial maximization horizontally and vertically to any opened window (if it's not a dialog). The policy should also remove the window decorations initially, too.
2. In amaroK open the Visualizations dialog and check any checkbox there.

Behavior:
You get a nearly completely black screen with only the visualization in the upper-left corner. It's about 100×100px (haven't measured it). If I fullscreenize the visualization window, I see it then fullscreen without blank space around it, but very lo-res. To get it to display properly, I have to Alt-RightClick anywhere while *technically* the vis is not fullscreen, but practically is due to window management policy set in KControl.

Expected behavior:
The vis realizes it's automaximized and fits the window appropriately without need for resizing.

Summary of this long report:
All of your windows should be set to autofullscreen. Launch any amaroK vis. The vis doesn't understand that its container is maximized and stays small. Resizing the vis window apparently sends some kind of message to the vis so it kinda wakes up when you resize the window and stretches to fill the window.