Cheat Engine

[Acclaimed]

I was trying to do something for a friend and as a joke I need a comically large weapon. How would I go about changing a weapon's size?

[Csimbi]

You'd need to find the model, find out how that model is read and processed.
Usually there's single float for 'scale', that's what you are looking for.
Once you got that, easy-peasy.

Problem is, it does not really change, mostly static so finding it will be quite a bother.

Try to find the player's weapon first. Equip different weapons and see what changes.
Eventually you should find either an ID or a pointer to the weapon.
The weapon usually comes from a template, so that will have an ID or a pointer, too.
Follow up with that and the weapon scale should be in sight.