In Leopard, you can execute a command on the terminal that allows you to set the Dock magnification outside the limits that you get using the slider in the System Preferences. To play with this, get a terminal window by going to Applications > Terminal. Once you have a terminal windows, type a command of the following format:

defaults write com.apple.dock largesize -int NEW_SIZE

where NEW_SIZE is replaced by a number between 16 and 512. You can use non-integer values. For reference, the sliders in the System Preferences panel allow you to move this value from 16 to 128. So, if you want maximum Dock magnification of 512 (it's *really* large), the command would look like this:

defaults write com.apple.dock largesize -int 512

Once you type that command and hit enter, you need to type this command (and hit enter) for the change to take effect:

killall Dock

Once you do that, you'll see the new level of Dock magnification that you requested. You can keep repeating this command to get a size you like, or you can go back to setting the value in System Preferences.