OS X is very un-customizable in this respect. The text size option only affects the icon text not menus etc.
There are three main ways you can improve the situation:
➊ There are several widescreen resolutions available. Not quite sure what they are on the 17" iMac G4 but on the 20" iMac G4 there is:
- 1680 x 1080 (native)
- 1344 x 840
- 1280 x 800
- 1024 x 768 (stretched).
There are also several smaller ones.
- 1440 x 900 (native) for the 17" iMac - try a lower one.
I would suggest this is the easiest way to ease the strain on eyes of small text.
➌ Use the Univeral Access Zoom function. This can be found on UA pane of System Preferences and allows the screen to be magnified. <apple><option/alt> and <+> or < - > to zoom in and out.