Making large icons

I have been fiddling with creating large Vista icons in GIMP, but I find
that whenever I zoom any icons I make larger than medium-sized, they
don't grow any more. What are the requirements for fully sizable Vista
icons? Does Microsoft have a web page about this? Does anyone happen to
know if GIMP is capable of creating large icons for Vista? (Everything
I've found on the Web so far suggests it can. I don't want to download a
dedicated icon-builder if I can help it, since I'm just fiddling.)

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David Trimboli
Windows Systems Analyst
Cold Spring Harbor Laboratory