One of the most basic parameters of an object placed at a camera is the object’s scale. Weapon should not be too big, otherwise it would cover the battlefield. Neither should it be too small, not to disturb perception of the scale of the surrounding. The best option is to stick to about one-eighth of the screen surface and not to exceed the fourth quarter.
An interesting approach towards the scale and settings is presented by Ryan Duffin in an article on animating weapon in FPP games.
When experimenting with gun placement for Medal of Honor: Warfighter, and surveying gun placement in other games, he found that the most pleasing weapon location corresponded to the spiral formed by the Golden Ratio as applied to the screen. By lining up all the game’s weapons according to this rule, he achieved a consistent and intentional look, instead of just placing them arbitrarily on the screen. source
The size of an object depends not only on the correctly adjusted proportions of space taken by a weapon in comparison with the screen space, but also on the weapon class.
When working on a given set, the correct relation between the scale and models should be preserved. Everybody knows that a pistol is smaller than a heavy machine gun as so on. The scale of weapon should be changed so as to widen the contrast between types of weapons. Players should get a clear message: The bigger the weapon, the more powerful it is.
Rage could be an example of a game where the size of weapon is proportional to its power.
The scale of hands in the FPP view should remain consistent with all the objects owned by a player. If, for some reason, we need to change the scale of hands for a particular weapon, the differences should not exceed about 15%.
2013. source: http://blog.wolfire.com/2013/04/GDC13-Summary-Animation-Bootcamp-Part-5-6