How can you control a Windows XP\2003 taskbar size via the registry?

How can you control a Windows XP\2003 taskbar size via the registry?

all the googling I've done only shows me how to lock or unlock it, how to hide it, or how to control the grouping setting. What I'm looking for specifically is, when you make the task bar one or two or three levels deep, or even zero levels which isn't the same as hiding it, where is that stored in the registry?