Check also the text margins in the quicklaunch buttons.
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That part is true...the rest,not so much,sorry 2 of 3.
taskbutton(and quicklaunch,systray)content margins determine height of the taskbar.Buttons and taskbar graphics can be damn near any size...they will grow or shrink to fit.Start button will just be cut off on the bottom if its taller than the height of the taskbar.
I often use mismatched image sizes for various reasons...no problem.It does simplify things if they are all the same.
I have a special design currently that needed to tile properly at all heights...single,double,triple,etc...It is 60 pixels tall...but as a single it is 30.An oversized Start button(vertically)only pushes the quicklaunch buttons down further on a vertical taskbar.
Taskbar height near as I can figure...is...16 pixels for icon,6 pixels of icon space and however much more you need in content margins to achieve your desired taskbar height.(total of 8 in my case,4 top and 4 bottom)Huge or tiny fonts may affect this formula but you can get your taskbar the right height regardless of your image sizes.