Is it possible to make the flashes from the guns bigger for night?
It would require to add code somewhere which considers the Sun's angular elevation. When below some threshold angle (say, -6 degrees, or -0.1 radians as the code uses), the effect for fire would switch to another new effect (say, FireNight.eff) created for this purpose. This effect could have its fire texture made larger, or brighter, or more numerous (textures that overlap get collectively brighter.)
Moreover, the classes already assign values for the illumination of the surroundings, lasting typically around 1/10 second. The two values control intensity and range. For nighttime firing, one or both of these values could be increased, along with the use of a more prominent fire effect outlined above.
I don't know if this code could be added to the individual weapon classes. I suspect more strongly that it would have to reside elsewhere. The scheme would likely be similar to the way aircraft gunfire muzzle flash effects are implemented, where there is a separation between effect folders named GunFire and GunFireDay.