There are lots of styles of Amiga icons, from the original icons in
Workbench 1.x,
2.x and
3.x, then later on
MagicWB, then the original
NewIcons package had a distinctive look of its own.
OS3.5 and 3.9 introduced the GlowIcons theme. Finally, third party patches like PowerIcons and AfA-OS have allowed Workbench to use PNG icons, which have allowed people to use beautiful true colour icons, though these are often taken from other Operating Systems (like mine - see my gallery). There are some icons sets which are designed specifically for the Amiga, like
these lovely ones, which are PNG icons and look a bit more Amiga-like.
So as you can see you need to decide which Amiga look you're after, then think about what you need the icons to do so you can pick the icons you want. Then you will need to scale them up to the correct size.
[EDIT]
Fixed and changed a couple of links. Also I think you missed McVenco's point. He was saying that you will never find 128x128 icons on an Amiga as that would be too big for most Amigas' resolution. Most icons designed for the Amiga are much smaller than that so you will need to scale them up, or redraw them larger.
[/EDIT]
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moto