Actually, the NSImage instance returned by -[NSWorkspace iconForFile:] contains all sizes* of an icon as NSImageRep instances. You can either access the reps directly and query them for the size you want, or you can ask the image to draw at a particular size.
* There will be up to four reps for the four possible Mac OS X icon sizes: 16x16, 32x32, 48x48 (not often used), and 128x128. If you want 64x64, you'll have to draw the 128x128 at half size.