Weapon sprites with a height of 3 pixels: SPAS-12, Knife
Weapon sprites with a height of 4 pixels: Desert Eagles, LAW, M79, Ruger
Weapon sprites with a height of 5 pixels: AK-74, Barrett, SOCOM, AUG
Weapon sprites with a height of 6 pixels: M249, MP5, Chainsaw
Weapon sprites with a height of 8 pixels: Minigun
It's pretty easy to check if you're not sure. I recommend doing so before resizing images. If you're too lazy, make the height 4 or 5 pixels, as most weapons will be able to handle this.