When i went to buy my surfboard the salesman tried to set me up with a used gun. It was in good conditions and quite long, but just so damn thin!
Guns are not good as beginers boards. They can be long, but they are very thin and have lots of rocher, wich does not help. A shortboard is much better, but then a shortboard even when big could be thin too and have lots of rocker...
In general the important is not just the lenght. You need a wide and thick board for easy padling. And the edges of the board should not be too puled down.