How do i make big icons and can i make them in LINUX too ?

I've been using Iconographer 2.0.2 and Photoshop 5.0 (which works fine in classic). I don't know of any Winodw Managers that can handle icons larger than 48x48 for Linux. Irix can deal with large Icons, but they have to be vector based (not raster) which OS X's are not.
IconComposer and icnsBrowser are in /Developer (assuming you've installed the OSX developer CD)

I use either Photoshop or Strata3d to make icons. If you drag a .psd file into IconComposer it gets imported with transparency and all, so you don't have to screw around with tiffs & alpha channels