Display size changes of its own accord

Which "model" & "generation" G5 iMac do you have?
Need this info so that the knowledgeable users can provide you with the proper troubleshooting solution(s).
http://docs.info.apple.com/article.html?artnum=301724-en How to identify your iMac
http://docs.info.apple.com/article.html?path=Mac/10.4/en/mh355.html Getting information about your computer

You can have this info displayed on the bottom of every post by going to 'My Settings' which is located in the column on the right under your name, and filling in the information asked for.

Thank you

In the meantime, a few basic suggestions:

Repair disk/permissions directly from your Tiger System Install DVD.
http://docs.info.apple.com/article.html?artnum=25751 About Disk Utility's Repair Disk Permissions feature

http://docs.info.apple.com/article.html?artnum=301283 Troubleshooting when there's no picture on the display

System Preferences>Universal Access>Seeing
Check your display settings.

System Preferences>Display
Check your Display & Color settings.

I was just presented with a 17in G5 (with camera) iMac today that does something similar. Namely the resolution of the screen would change at somewhat random intervals to lower resolutions. After much fiddling, resetting PRAM, checking disk and permissions etc. I wasn't really able to find a resolution. It seems like the iMac thinks it is running on an external display.

Thinking along those lines I found that the "Detect Displays" (in the displays control panel) button finds a VGA display and now presents a VGA Resolutions panel. Changing the settings here changes the iMac display though the resolutions offered are more numerous and mostly different than those offered through the regular Resolutions list. In fact I could no longer choose anything from the regular iMac Resolutions list as it was always below the new VGA dialog. The menu list of recent resolutions did seem to allow me to choose the old iMac default resolution but nothing changed.

Choosing a suitable VGA resolution seemed to give good results and persisted across a reboot. I'll have to wait and see if the problem is "fixed" by this somewhat strange procedure. The machine was taken home again by the owner but I should hear whether it still works in the long run.

I have exactly the same problem with my iMac G5. Apparently it thinks it has two displays sometimes and it randomly switches between the two. Another post that describes this problem is

http://discussions.apple.com/click.jspa?searchID=4556063&messageID=6287416

No solution has been proposed yet. If anyone has a clue, please post.

An addendum: The system log shows the message

removeDisplayMapping: _CSGUnmapFramebuffer returns -536870206

a lot. I don't know if this correlates to the problem.

"I have tried to verify permissions but don't really know what Im doing there."

If the Article was not clear, here are step by step instructions:
When troubleshooting, disconnect all devices/peripherals from your computer except for the keyboard & mouse.

Save and/or quit your applications & doucuments

Go to Applications>Utilities Folder

Click Disk Utility

Click the Volumn (HD) that you want to check.

Click the Repair Disk Permissions button. (Don't worry about verifying)

At the bottom of window when the repairs have finished, you will see the “most important thing”:
Permission Repairs Complete
The previliges have been repaired or completed on the selected volumn.

Restart your computer (Optional).

Go to Applications>Utilities>Disk Utility and select your volume and then select Repair Permissions.

**Same directions are also in the OS Mac Help Menu.**

Did you try the display troubleshooting solutions listed in my previous post in this thread?

Here is another one which I forgot............
System Preferences>Display>Color

Click on the purple button with the black ? mark in the middle. It's located at the bottom right hand corner.

The help window will pop up. At the very bottom of the page click on the Color Sync Utility link.

Profile First Aid window should pop up. Follow the instructions & run the test.

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If none of the above works out, you may need to take your comp to your local AASP.
The iSight models are NOT user repairable except for installing & removing memory & these same models are NOT included in any of Apple's Extension Repair programs.
Hopefully, you had the piece of mine of purchasing AppleCare.