Hey Brecht,
The more time you spend diving with decent fish, the better your judgement becomes. When you shoot a fish, measure the length (as suggested by Landshark in the thread) and the weight (on land of course).
I like what the other guys say regarding your "gut feel". If you're questioning it, leave it.
Diving with guys that are more experienced than you is probably a step in the right direction. Stick together, Work together, glean as much as you can. Maybe your mate shoots a size fish... Look at it in the water. Get accustomed to the shape and size in this awesome environment we play in...
That's all from me...
Happy hunting
L