To me, the longer the better (so long as it will fit in the car). Pick one with a natural side branch at the top. Then it is perfect for reaching fruit, fishing things out of streams etc. while you are out walking. For a shorter stick the hard part is finding one with a top that is comfortable to your hand or has a top that a comfortable handle can be easily attached to.
And at the other end: there comes the problem of what to use for a ferrule or end stop. It seems all the nice sticks I find come with the ends needing an odd size of pipe of which I have no convenient offcuts lying around. (And, if you are using the traditional cabbage stalk, this is important because it is hollow so there isn't much wall thickness to play with.)
S