“How close to the edge of my shelf can I place a large piece?”
It depends in one sense how thick the piece is. A 6mm piece that maintains the same footprint after firing as before, does expand beyond that footprint by about half a centimetre during the firing, so it would be safe to have a full centimetre space to the edge. Thicker pieces will need more space – 9mm will need about two centimetres to accommodate the expansion at the top temperature.
The real answer to this question is: When you know the heat characteristics across your shelf, you will know how close you can go to the edge for a relatively large piece.
This Bullseye Tech Note number 1 tells you how to test the variations of temperature across your kiln. - http://www.bullseyeglass.com/methods-ideas/technotes-1-knowing-your-kiln.html
The objective in cooling glass is to have less than a 5C difference in temperature over the whole of the glass piece – top to bottom, and side to side.
If you have greater differences in temperature than that at the edges of your kiln shelf, you need to avoid placing large pieces in the danger area. Small pieces will not suffer by being close to the edges as their temperature differentials will be small.
I have found that the temperature differential in one of my kilns is great enough at the edges that I cannot have the edge of a relatively large piece of glass nearer than 50mm (2") from the edge.