In SkinStudio, the "Explorer" view->Taskbar->Start Button. In the far right hand side are your "coloring" and "section" tabs. With "section" tab activated (highlighted, in use...), you should have
Start Button (image)
Windows paint the flag
sizing margins
content margins
glyph image
margins
painting options<-
Expand "Painting options". Then you have
Start Button custom coloring
mouseover effect
draw method
Simple Draw method
Hope this helps. If you are wanting a start button that overlaps the taskbar, I am not sure how to do this. Kol and Mike Bryant are the only authors that I know of that have made skins with start buttons larger than the taskbar.