You'll have to pattern your gun really. Don't get your head set on using one particular size shot (there is nothing magical about 00). Shoot several different sizes of shot with different choke settings (if you got interchangable chokes) and see what shoots best. Barrel length does not matter one bit. Shorter barrels just (1) make a little more noise, and (2) are handier to handle.
Having said that, it sounds like you've already got the shells, and just want to know what choke tube to screw in. I'd start with modified and see where it goes from there.
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John 3:16 (NKJV)