I see nobody responded to this earlier. I have successfully connected a 15" LCD to a 17" motherboard. The connectors are the same. There is a difference between the two PSUs, however. The 17" models provide more power to the LCD because the large screen has more backlight. You may find that connecting a 17" display to a 15" mobo will sooner or later cause problems with overheating or other symptoms in the PSU.
You might also find that a 17" mobo will not fit in a base designed for a 15" system. The larger heat pipe arrangement uses a different casting in the base to make the essential heat-sinking contacts.
The top sections of the domes are interchangeable, however.
Keeping the PSU free of dust is very important in these models. Some people recommend occasionally applying a vaccuum cleaner to the fan outlet at the base of the neck to suck out any fluff that may have gathered. Also make sure the iMac sits on a hard, flat surface, not any kind of fluff-generating mat or cloth from which fluff can be sucked in and gather in the PSU, just below the fan.
The actual display scale management is handled between the video chip and the OS, which will respond to the display identification information that the LCD unit firmware feeds to the OS.
The video chip gets quite hot and it is very important that when you reassemble a G4 iMac you properly tighten the base screws so that the graphic chip is in firm contact with the heat exchange pad that links it to the aluminium base, because that is its heatsink. I blew one once, just by trying to test it outside the base for a couple of minutes. Blown graphic chips and failed PSUs are the most common failures for these systems, both usually due to overheating through poor heatsink contacts or dust accumulation.
Rick
