As far as largest handgun cartridge goes, it would be the Pfeifer Zeliska .600 Nitro Express.
If you're referring just to size of the firearm itself, that can be a bit ambiguous. Take, for example, the AK style "pistols" manufactured which are classed as pistols simply because they lack a buttstock (otherwise, they'd have to be NFA regulated SBRs, having a barrel length of under 16 inches).
The biggest gun is a SPG gun the Monster which is a german rail gun mounts a 800mm mortar
desert eagle
The .950 JDJ is the biggest rifle The biggest hand held gun is the 600 Nitro Express Pistol
You will have to define if you are talking about hand held or crew served.
.500s&w
The Amazon is the worlds biggest rain forest
The Catholic Church is the biggest charity in the worlds.
the worlds biggest yoyo is about the size of a hobo
The Potato Gun
The worlds biggest island is clearly Green Land its PNG
The worlds biggest selling drink is both Milk and water.
The worlds biggest tin sheep here