Measurements for cut out door

[Trish]

I want to cut out an area in the barn so the goats can out out to a fenced in chain link pen area if they need to (they free range during the day though).  Does anyone have any suggestion on the size of the cut out so full grown nigerians can exit.  I also don't know what kind of  "flap" (for lack of a better word) or plywood to secure  the cut out  from external elements rain, snakes etc.  Also better doing it from the inside or outside? Pics would be helpful.

All suggestions would be appreciated.

Thanks