If your 128x128 icons aren't big enough for you, you can increase the magnification of the dock to really big. Type this into Terminal:
defaults write com.apple.dock largesize -float 256.000000; killall Dock
BTW, you can change "256" to any number bigger than the default dock icon size. I don't know why the size has to be a float, but that's the way it started out, so that's the way I left it. Unfortunately, none of my apps has a 256x256 icon, so all the icons simply look blurry. But they're really big!
defaults write com.apple.dock largesize -float 256.000000; killall Dock
BTW, you can change "256" to any number bigger than the default dock icon size. I don't know why the size has to be a float, but that's the way it started out, so that's the way I left it. Unfortunately, none of my apps has a 256x256 icon, so all the icons simply look blurry. But they're really big!