Heres how I do it.
1. create model
2. get the green box around it.
3. use that box for material size or add to top and or xy to your preference
4. use the rhino analyze tools (length, distance) to measure the green box
5. now you know your stock size. (madcam already knows the stock size, it is just for the operators information
another example (task)
cut parts out of 4 x 8 sheet stock.
1. create a 4 x 8 surface in Rhino
2. orient it where you want it to be in relation to 0,0
3. draw curves within the x, y boundery of this surface representing the parts to cut
4. select the 4 x 8 surface when madcam asks and you have a virtual 4 x 8 for your material
Hope this helps