no offense buddy, but this is really a dumb question. every jump is different and has different amounts of pop. i know a 30 foot jump at my mountain that i have seen doubles off of, and i have also hit a 50 footer with no pop that wud be nearly impossible to dub off of. also u need to tell us what kind of dub u are thinking. if its just a straight dub back then u need a smaller jump then u wud need for a dub cork 10 or something. u gotta give us more details.
my advice to you is get comfortable doing loads of tricks on very big jumps. once u get very comfortable with a big jump you will know if it is big enough for a double. i have never done a dub but i have hit jumps where i know its possible.