Please read these articles then
The scripting guides, and the GS1 to GS2 should definitely be read.
http://graal.net/index.php/Creation/Dev/GScript
They should be quite helpful.
Edit:
You need to have a variable that holds how big the fish was first.
Then you can do:
PHP Code:
player.chat = "I caught a " @ int(variable) @ "!";
Substituting variable with what you picked of course.