Are you referring to when he has long hair and a pony tail in the back like this
Assuming you are planning on giving it a bit of a trim Angela is a good length to start with. If you don't cut it it'll be too long once you straighten it (a little past the shoulders I think).
Otherwise go for a wig that is between 11"-12" long (from centre part) or about 17" (from the front of the hairline to the bottom back of the wig). A long bob like this one
would be good (though this one doesn't come in purple! which is a shame because amphigory's Lily #60 is a great colour for Trunks).