Personally I wouldn't go with less than 80 oz, if you can fit it. If size is an issue you can always use a 60 and a 24. I have a 16 oz Dubro mounted to the top of the 71 oz stock tank in my H9 MB-339 with a Xicoy 85. I like to land with at least a 1/4 tank after a normal 6 minute flight, that way if there are issues with landing (like gear issues, or having to get in line at a big event) I don't have to worry about a dead stick.