Question can't be answered as written. One hand or two hands?
M2 can be hand held, but, that is the exception. MP5 in 10mm is along the lines of what you might be asking about.
A hand gun.
colt
The .950 JDJ is the biggest rifle The biggest hand held gun is the 600 Nitro Express Pistol
Most likely it is a Hi-Power if it is a 9mm.
mg 42
You will have to define if you are talking about hand held or crew served.
They could be designed to, but they don't.
One of the largest, based on sheer mass and size is the aptly named "Pig"; or the M60. While not the largest "hand held" gun ever, it is among the largest hand-held, single person (not crew served) weapons that had been in common use. If your looking for length, the Lhati L-39 is the longest single-person weapon in use with a military that has the possibility of being hand-held.
A machine gun held in the hand called the GAU 51
The Potato Gun
Not actually a gun, it is a hand held radar unit that can measure the speed of moving objects, such as cars, baseballs, etc.