Help with flue outlet collar

[stovepipemike2]

I am in the process of gathering metal together for a small wood burning portable stove to take camping etc [outdoor use] The firebrick burn area is 8 in X 16 in, The stove will be 16 in, high. Yes it is small , basically a curiosity to fry an egg or make cowboy coffee on. A way to extend the wood stove season if you will. How do I determine what size flue outlet is adequate for the job. Does it matter if the air comes thru the firebox from the top as a downdraft unit or thru a front loading door. Thanks for your thoughts. Mike