You don't want you glass overhanging, for a couple of reasons:
1 - your glass will have to be just sitting there, you won't be able to use any kind of glass clips to hold it onto the lid of you cabinet - mine sits there, and I ahve to pick it up and move it out of the way EVERY TIME I want to open teh lid - annoying.
2 - the overhanging edges will be nowhere near as strong as the bits supported by the lid.
I reckon order it exactly to size, or a tiny bit smaller, and sand the edges of your lid to fit the glass (coz once it's cut, there's no resizing your glass)
as for the thickness, I just asked a glass guy to give me a piece of glass that he would use for a coffee table - strengthened safety glass is a must with your wrists so close
