The biggest caliber an American civilian shooter can easily get is .50 BMG. This is the round that .50 Browning machine guns use.
There are bigger calibers out there. Some elephant rifles use cartridges that fire a .60 caliber bullet. If you have the right paperwork and permits from the federal government, you can buy a small cannon and shoot 30mm or larger rounds. But among "common" rifle and pistol calibers, the .50 BMG is king of the hill.
.45 Caliber is the largest caliber Glocks chamber.
50 caliber
You will haver to define if "biggest" means caliber or overall size.
.17 to 20x110 Hispano
.500s&w
In guns including firearms, caliber or calibre is the approximate diameter of the barrel and by extension the projectile used in it, measured in inches or millimetres. The 20mm caliber ammo is the biggest one in production.
The .950 JDJ is the biggest rifle The biggest hand held gun is the 600 Nitro Express Pistol
There has been a revolver made in caliber .600 Nitro Express. This was an elephant rifle cartridge. As a firearm, it is a curiosity, and not a practical shooting firearm. There are also unconfirmed reports of a Maadi bolt action pistol in caliber .50 BMG.
It depends on whether you mean legally, or what is the biggest handgun available, and whether you mean handgun size, or caliber size. The largest commonly available caliber is .45 ACP, however, .50 AE is also fairly common. That said though, there is also a handgun available in .600 NE. Most states don't have restrictions on caliber, however, some may.
The BATFE web site has information that can assist you in answering your question.
Largest standard caliber is likely the S&W .500 Magnum, but revolvers HAVE been built in .600 Nitro Express.
No such thing. You can have small caliber but not low caliber.